Ukraine boosted its armed forces by 100,000 troops over three years with a decree signed by Ukrainian President Volodymyr Zelensky on Tuesday, and diplomatic contact will continue in the coming days on the rising tensions between Russia and the West over Ukraine.

The tensions have been building for weeks, with Washington accusing Moscow of preparing an imminent invasion of Ukraine. Russia denies any plans to invade but is demanding that Ukraine never be allowed to join NATO and a series of other security guarantees against the U.S.-led military alliance's expansion.

U.S. contact with Russia continues

U.S. Secretary of State Antony Blinken is to speak by phone with his Russian counterpart Sergei Lavrov mid-morning on Tuesday Washington time, AFP reported.

Ahead of the call late Monday, Moscow sent a letter to Washington outlining its views, in response to written communications laying out the two sides' positions that were exchanged over the past two weeks.

Blinken and Lavrov held face-to-face talks January 21 in Geneva, and since then the two sides have exchanged written responses in order to try to make their positions clear.

Zelensky and U.S. President Joe Biden discussed diplomatic efforts on de-escalation of the crisis in and around Ukraine during a telephone conversation several days ago, which marked the second conversation between the Ukrainian and the U.S. leaders since the beginning of the year.

UK warns Russia to 'step back'

British Prime Minister Boris Johnson was due to meet Zelensky later on Tuesday.

"We urge Russia to step back and engage in dialogue to find a diplomatic resolution and avoid further bloodshed," Johnson said in remarks released ahead of his arrival. 

"As a friend and a democratic partner, the UK will continue to uphold Ukraine's sovereignty in the face of those who seek to destroy it." 

Meanwhile, a phone call between Johnson and Russian President Vladimir Putin has been rescheduled for Wednesday afternoon.

Italy calls for 'de-escalation'

Italian Prime Minister Mario Draghi called for a "de-escalation" of tensions over Ukraine in a telephone call Tuesday with Putin.

They agreed on a joint commitment to a sustainable and lasting solution to the crisis, and the need to rebuild a climate of trust, according to the PM's office.

Putin, for his part, accused Ukrainian authorities of violating obligations to settle a conflict with Moscow-backed separatists, the Kremlin said.

UN stresses dialogue to defuse tensions

UN Undersecretary-General for Political and Peacebuilding Affairs Rosemary DiCarlo on Monday called for dialogue to defuse the current tensions between Russia and Ukraine, and stressed the UN secretary-general's appeal to all concerned to take immediate steps to de-escalate tensions and continue on the diplomatic path.

"Accusations and recriminations among the various actors involved in the ongoing discussions have created uncertainty and apprehension for many that a military confrontation is impending," she said.

China's UN ambassador on Monday also called for quiet diplomacy instead of megaphone diplomacy on the tensions between Russia and Ukraine. 

In a procedural vote, China and Russia voted against a Security Council open meeting on Ukraine. The meeting went ahead as 10 other members of the council voted in favor. 

(With input from agencies)