NATO Secretary-General Anders Fogh Rasmussen has expressed concern about a large increase in Russian troops and armaments along Ukraine's eastern frontier following its annexation of Crimea.
"I can assure you that we are very much concerned about the Russian military buildup along the borders of Ukraine," Rasmussen said. "We are, as an alliance, focused on providing effective deterrence and defense. And all NATO allies can be assured of our determination to provide effective defense."
Speaking in Brussels on March 25, Rasmussen said NATO is ready to defend all its members and offer assistance to Kyiv.
"We are also in the process of discussing with Ukraine how we can enhance our support for Ukraine," Rasmussen said.
In The Hague, the United States accused Russia of violating its commitments under the 1994 Budapest Memorandum by seizing and annexing Crimea from Ukraine.
In a joint statement issued at a nuclear security summit, the United States and Ukraine said Russia's actions in Crimea "undermine the foundation of the global security architecture and endanger European peace and security."
G7 leaders also warned Russia it faces economic sanctions if it continues to destabilize Ukraine after annexing Crimea.
The G7 leaders issued the statement after an emergency meeting in The Hague on March 24 on the sidelines of the nuclear security summit.
They also said they were suspending cooperation with Russia in the G8 until Moscow "changes course." They also canceled a planned G8 summit in Sochi in June.
Russian Foreign Minister Sergei Lavrov -- also in The Hague -- dismissed the G8 boycott.
Meanwhile, lawmakers in Kyiv approved the resignation of acting Defense Minister Ihor Tenyukh over his handling of the Crimea crisis and approved the nomination of General Mykhaylo Koval as his replacement.
Tenyukh submitted his resignation on March 25 amid criticism that he had failed to give timely orders to Ukrainian military units in Crimea during its occupation and annexation by Russia.
Ukraine's acting President Oleksandr Turchynov had requested that Tenyukh be relieved of his duties.
In Berlin, the New York-based watchdog Human Rights Watch (HRW) says two Ukrainian political activists were beaten and one tortured by pro-Russian forces in Crimea.
HRW says Andriy Schekun and Anatoliy Kovalsky -- who had gone to Crimea to organize pro-Ukraine rallies ahead of the March 16 referendum on Crimea's status -- were detained, interrogated, and beaten for 11 days before being released.
HRW said Schekun had received electric shocks, was tied up, and often kept naked.
Hugh Williamson, HRW's Europe and Central Asia director, said "these horrendous arbitrary detentions and the allegation of torture in Crimea urgently demand a thorough investigation."
The fates of up to six Ukrainian military officers remain unknown following the takeover of Crimean bases in recent days by Russian forces.
The commander of a Ukrainian air base in Crimea, Colonel Yuliy Mamchur, was captured last weekend after Russian forces stormed the Belbek air base.
Mamchur's aides believe he is being held in the Russian Black Sea Fleet's home port of Sevastopol.
HRW says five other Ukrainian military officers and three activists also remain missing in Crimea.
In the western Ukrainian city of Rivne, a leader of the ultranationalist Right Sector movement, Oleksandr Muzychko, was killed overnight on March 24-25.
First Deputy Interior Minister Vladimir Yevdokimov said he was fatally wounded while security forces were trying to detain him during a special operation.
An international arrest warrant was issued for Muzychko this month for his alleged killing of Russian military personnel during the Chechen war in the mid-1990s.
Based on reporting by Reuters, dpa, AP, and Interfax