Russia has declined to participate in an OSCE meeting on Saturday to explain its unusual military buildup on Ukraine's borders and in the occupied Crimea, the U.S. Mission to the OSCE reports.

"We regret that Russia did not avail itself of this opportunity to address concerns and reduce risks under the Vienna document," the Mission tweeted.

The corresponding meeting was earlier called on Ukraine's initiative.

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Read alsoZelensky: Ukraine, Turkey have "common vision" of security in Black Sea regionRussian military buildup: Background

On March 30, 2021, Commander-in-Chief of the Armed Forces of Ukraine, Colonel-General Ruslan Khomchak said Russia had deployed 28 battalion tactical groups along the state border of Ukraine and in the temporarily occupied territories. It is also said to be planning to bring up to 25 battalion tactical groups under the guise of preparing for military drills. Such actions pose a threat to Ukraine's military security.

According to the Armed Forces of Ukraine, enemy troops are reinforcing their advanced units with reconnaissance teams and sniper pairs, involving Russian army instructors in personnel training. Russia-led forces' artillery units are reportedly on full alert in certain districts, including villages and towns in occupied Ukrainian territory.

On March 31, 2021, The New York Times reported Russia was pulling 4,000 troops to the border with Ukraine.

Footage was shared on the Internet, showing a train with Russian military hardware en route along the Kerch Strait bridge to occupied Crimea.

The U.S. European Command raised its alert status to the highest level after fighting had resumed between Russia-led forces and Ukrainian troops in Donetsk region.

The U.S. Department of Defense said it was "aware of Russian troop movements" on Ukraine's borders and was concerned about recent escalations of Russian aggression in eastern Ukraine.

Read alsoFive states condemn Russian military activity near Ukraine's bordersThe United States, finding reports of Russian military movements on Ukraine's border credible, asked Moscow to explain the "provocations" and is ready to engage on the situation.

The Vienna Document: Memo

The Vienna Document 1999 is the most important confidence- and security-building measure (CSBM) in the OSCE area.

The Vienna Document requires participating States to:

Provide each other with information about their military forces annually, including about manpower and major conventional weapon- and equipment systems, as well as deployment plans and budgets;

Notify each other ahead of time about major military activities such as exercises;

Accept up to three inspections of their military sites per year. Some sensitive areas are excluded;

Invite other States to observe certain activities. It also encourages States to permit journalists from all participating States to cover the activities; and

Cconsult and co-operate in case of unusual military activity or increasing tensions. The Vienna Document encourages participating States, for example, to voluntarily host military visits to dispel concerns.