London - British Prime Minister Boris Johnson, due to visit to Kyiv on Tuesday, vowed to uphold Ukraine's sovereignty. The visit is a part of the West's diplomatic efforts to stop a possible Russian invasion, though denied by Moscow.
Russia, which seized Crimea from Ukraine in 2014 and backs separatists in the east of the country, is demanding security guarantees including a promise NATO will never admit Kyiv.
The United States has said there is little chance of Ukraine joining soon but that the country should decide its own future.
The countries, both Russia and the United States, during the United Nations Security Council meet on Monday over the Ukraine troop buildup, used the international forum to label each other as "provocative".
Johnson, is due to meet Ukraine's President Volodymyr Zelenskiy as he focuses on Britain's global role in the world, which he has much touted since Brexit.
Johnson is due to discuss with Zelenskiy what strategic support Britain can offer to Ukraine.
London has supplied defensive weapons and training personnel to Ukraine, though ministers have said that the deployment of combat troops is unlikely.
On Monday, the United States and Britain said they were prepared to punish Russian elites close to Russian President Vladimir Putin with asset freezes and travel bans if Russia enters Ukraine.
Poland has said it had offered neighboring Ukraine tens of thousands of munitions and was awaiting a reply.
The United States ordered the family members of its government employees in Belarus to leave as it warned against travel there amid tensions over Ukraine.
A call between Johnson and Putin, which had been planned for Monday, could take place on Tuesday, according to Downing Street.
French President Emmanuel Macron and Putin agreed during an exchange on Monday to maintain a dialogue on implementing the Minsk agreements regarding Donbass, a region of eastern Ukraine where Moscow has backed separatist fighters.
Russian Foreign Minister Sergei Lavrov is due to speak by phone with U.S. Secretary of State Antony Blinken.
The State Department said on Monday it had received a written follow-up from Russia after Washington submitted responses last week to Moscow's demands over security arrangements on the continent.
Kremlin spokesman Dmitry Peskov had previously said that U.S. and NATO statements describing Russia's main demands as unacceptable did not leave much room for optimism.
Russia has placed an estimated 100,000 troops, tanks, artillery and missiles near Ukraine's frontiers, but denies it plans to invade the former Soviet republic, which also borders the European Union.
The UK is also sending weapons to Ukraine and offering extra support to nearby Nato allies. The country also has more than 900 military personnel based in Estonia and more than 100 in Ukraine as part of a training mission, while a light cavalry squadron numbering about 150 is deployed to Poland.
Nato's 30 members include the US and UK, as well as Lithuania, Latvia and Estonia - former Soviet republics which border Russia. Moscow sees Nato troops in eastern Europe as a direct threat to its security.
-Reuters, BBC