The United States has accused Russia of supporting separatists in eastern Ukraine with military hardware, including tanks.
A "fact sheet" released by the U.S. State Department on July 14 said Washington had "no evidence that Russia’s support for the separatists has ceased."
"In fact, we assess that Russia continues to provide them with heavy weapons, other military equipment and financing, and continues to allow militants to enter Ukraine freely."
According to the State Department fact sheet, Moscow continues to accumulate military equipment -- including a type of tank no longer used by Russian forces -- at a deployment site in southwest Russia.
"We are concerned much of this equipment will be transferred to separatists, as we are confident Russia has already delivered tanks and multiple rocket launchers to them from this site," the State Department said.
NATO said Russia had increased its soldiers on the frontier to some 10,000-12,000, up from less than 1,000.
A NATO official said the larger number of troops is "not a step in the right direction" but rather "a step away from de-escalating the situation."
President Petro Poroshenko held an emergency meeting with his leading security officials in Kyiv on July 14.
Afterward he said that "information has...been confirmed that Russian staff officers are taking part in military operations against Ukrainian forces."
Poroshenko also said there is a need for tactical changes by Ukrainian forces to strengthen the defense of the border and protect people living near it.
Ukrainian Defense Ministry Valeriy Heletey said a Ukrainian AN-26 military transport plane had been shot down on July 14 by a rocket that was "probably" fired from Russian territory.
Heletey said on July 14 that the plane was flying at an altitude of 6,500 meters, which he said is too high for weapons being used by the pro-Russian separatists that are fighting Ukrainian troops.
Ukrainian officials said some members of the crew -- which is reported to number eight people -- have been in contact with Ukrainian military officials.
Kyiv officials dismissed a separatist official's claim of responsibility for downing the large plane.
There is no word on casualties, though an AFP reporter said there were charred human remains at the crash site of a military plane believed to be the AN-26.
A separatist spokesman said some of the survivors of the crash had been captured and were being interrogated.
Meanwhile, Russia said it will allow members from the Organization for Security and Cooperation in Europe (OSCE) to monitor two checkpoints on its border with Ukraine.
Andrei Kelin, Russia's OSCE envoy, made the announcement in Vienna.
Russian Foreign Minister Sergei Lavrov sent a letter on July 14 requesting that OSCE observers visit Russian border towns affected by the fighting.
Speaking with Swiss President Didier Burkhalter, chairman of the OSCE, Lavrov called for the resumption of talks to negotiate a cease-fire.
Poroshenko had a unilateral 10-day cease-fire but abandoned it when rebels would not lay down their arms and return captured border posts.
After a four-way telephone call with the French, Ukrainian, Russian and German foreign ministers late on July 14, Germany's Frank-Walter Steinmeier said all parties agreed to keep working toward a sustainable, bilateral cease-fire.
He said Russia had reiterated assertions it would use its influence on the rebels and has agreed to allow the Ukrainian embassy access to a detained female Ukrainian pilot.
Steinmeier also said the OSCE mission is also preparing to use drones to effectively monitor "the border control activities" to prevent the flow of fighters and weapons into Ukraine.
On the battlefield, the Ukrainian Defense Ministry said on July 14 that its troops had retaken several villages around Luhansk -- including Metalist, Oleksandrivsk, Bile, and Rozkishniy -- and had created a corridor to the Luhansk airport.
In related news, Poroshenko and Polish Prime Minister Donald Tusk have discussed plans to form a joint military brigade including the two countries and Lithuania.
Based on reporting by AP, Reuters and dpa