NATO Secretary-General Jens Stoltenberg told Moscow to withdraw some 1,000 items of Russian military equipment from eastern Ukraine, as diplomats said the UN Security Council is to hold an emergency session on Ukraine on February 27.
Stoltenberg, speaking in Rome on February 26, said he is cautiously optimistic the cease-fire agreed to two weeks ago between pro-Russian rebels and Ukrainian forces could be maintained.
He also said any attempt by pro-Russian rebels to expand the territory they hold in eastern Ukraine would be "unacceptable," and urged Russia to "stop supporting separatists."
Diplomats were quoted by AFP news agency as saying France and Germany requested the meeting of the 15-member council as Ukraine's military began withdrawing heavy weapons from the frontline and fighting with pro-Russian rebels appeared to subside.
Vitaly Churkin, Russia's envoy to the UN, told TASS news agency that the session will be held "behind closed doors."
It will be the first session of the Security Council since it met on February 17 to endorse the cease-fire deal reached in Minsk between Russian President Vladimir Putin and the leaders of Ukraine, France and Germany.
Stoltenberg, speaking in Rome on February 26 after talks with Italian Prime Minister Matteo Renzi, also said any attempt by pro-Russian separatists to expand the territory they hold in eastern Ukraine would be "unacceptable."
He said he is cautiously optimistic the cease-fire agreed to two weeks ago between pro-Russian rebels and Ukrainian forces could be maintained.
He said he "welcome the recent indications of reduced fighting in Ukraine and we hope that the cease-fire now is finally going to be respected."
Stoltenberg said Russia has in recent months transferred more than "1,000 pieces of equipment -- tanks, artillery, and advanced air-defense systems" into Ukrainian territory held by the rebels.
He said Russia must "stop supporting separatists."
Stoltenberg spoke as the Ukrainian military said it was starting to withdraw heavy weapons from the front line on February 26 after reporting that there were no combat deaths for a second straight 24-hour period.
The announcement raised hopes that a European-brokered cease-fire deal agreed Minsk two weeks ago could be taking hold.
"Implementing the agreements reached in Minsk on February 12, Ukraine is beginning the withdrawal of 100-mm guns from the line of contact today," the military said in a statement. "This is the first step in the pullback of heavy weapons."
Kyiv had been unwilling to begin its withdrawal because of what it said were persistent rebel attacks in violation of the cease-fire that took effect on Febrtuary 15 under the Minsk deal.
The agreement was brokered by the leaders of Germany and France in a bid to end a conflict that has killed about 5,700 people in eastern Ukraine since April.
It commits the sides to withdraw weapons with a caliber of 100 mm or more to create a security zone at least 50 kilometers wide -- and up to 140 kilometers for some missile systems -- within 14 days.
Pro-Russian separatists earlier said they had begun withdrawing their weapons, but their claim has not been verified by monitors.
The U.S. State Department noted February 26 that there had been a "slight improvement" in the fighting in Ukraine, but insisted the ceasefire was still being violated.
"There is a reduction we've seen over the past two days," State Department spokeswoman Jen Psaki said when asked if the truce was still being broken.
"But there are still violations... I wouldn't go so far as to call it a positive step," Psaki told reporters.
Hours before Ukraine's announcement, Russia called Kyiv's demand for a full cease-fire before it could start the pulback "ridiculous."
Russian Foreign Minister Sergei Lavrov said at a news conference on February 26 that "Everyone understands that there isn't an ideal truce and an ideal cease-fire regime."
Hitting back at Western accusations that Russia and the rebels have flagrantly violated the Minsk deal, Lavrov lashed out at U.S. Secretary of State John Kerry and other officials over warnings that new sanctions could be imposed over Moscow's interference in Ukraine.
Lavrov spoke after Kerry told U.S. lawmakers on February 25 that "neither Russia nor the forces it is supporting have come close to complying with their commitments" under the deal.
He said such remarks reflected "a lack of desire" in "the United States and the EU to achieve what we agreed upon in Minsk on February 12."
The cease-fire came into effect on 15 February but the rebels seized the strategic town of Debaltseve just days later.
Kerry accused Russian President Vladimir Putin of destabilizing Ukraine through "land grabs," and said: "We are poised yet to do another round [of sanctions] potentially, depending on what happens with [the cease-fire] in these next few days."
About 5,700 people have been killed and hundreds of thousands have fled their homes since the fighting began in April, one monthg after Russia annexed Ukraine's Crimean peninsula.
In Washington, former Georgian President Mikheil Saakashvili met with Assistant Secretary of State Victoria Nuland this week to discuss the Ukraine crisis.
Saakashvili, an adviser on reforms to Ukrainian President Petro Poroshenko, said this week he is meeting with dozens of officials in Washington to advocate for U.S. arms supplies to Ukraine in its standoff with pro-Russian separatists.
A State Department official told RFE/RL in emailed comments that in the February 24 meeting, Nuland "expressed our continued commitment to supporting the Ukrainian government’s efforts to root out corruption and promote economic development."
U.S. Senator Benjamin Cardin (Democrat-Maryland), who met with Saakashvili on February 25, told RFE/RL that he believes the former Georgian president’s visit to Washington has helped "underscore the importance of Ukraine in having the capacity to defend its borders."
Cardin also warned that if Russia continues to violate its international agreements, "the pressures that have been put on [Russia] through sanctions will only get stronger."
Saakashvili's visit to Washington came as top U.S. defense leaders told Congress that the Obama administration is still debating whether to provide lethal defensive arms to Ukraine.
U.S. General Philip Breedlove, the top NATO commander, told the House Armed Services Committee that he has laid out military options the administration could consider for Ukraine, ranging from sending small arms to more sophisticated weapons that would take longer to arrive and require extensive training.
Breedlove and Christine Wormuth, defense undersecretary for policy, told lawmakers they are worried most that Russia might move on to destabilize non-NATO countries such as Montenegro or Moldova and expand its military assault into other portions of Ukraine.
German Chancellor Angela Merkel, speaking on February 26 at a news conference with Romanian President Klaus Iohannis in Berlin, voiced "hope" that Moldova will not have the same fate as Ukraine.
She said, "We are now in the process of ensuring the cease-fire in Ukraine that would hold over a longer period of time so we can proceed with what we had agreed at the Minsk meeting."
Meanwhile, the European Union and the United States issued fresh calls on Russia to release hunger-striking Ukrainian pilot Nadia Savchenko.
The statements came after a Moscow court upheld a ruling extending the pretrial detention period for Savchenko, who has been on a hunger strike for 76 days, until May 13.
A spokeswoman for EU foreign policy chief Federica Mogherini, Maja Kocijancic, condemned the decision on February 26 and said that "Russia bears a responsibility for [Savchenko's] very fragile health."
She called for Savchenko's urgent release "on humantarian grounds."
In a statement on February 25, U.S. State Department spokeswoman Jen Psaki said the United States "deplores[Savchenko's] ill-treatment and is deeply oconcerned about her deteriorating health.
Savchenko was captured by the rebels in June and taken to Russia, which has charged her with involvement in a mortar attack that killed two Russian journalists.
Savchenko denies the charges and says Russia has no right to hold or prosecute her.
With reporting by Reuters, AP, AFP, dpa, and BBC