With tens of thousands of Russian troops massed on the Ukrainian border, the British and Polish prime ministers were to meet with President Volodymyr Zelensky in the Ukrainian capital.

Both countries have been strong supporters of Ukraine and before he left Britain Prime Minister Boris Johnson vowed to "uphold Ukraine's sovereignty in the face of those who seek to destroy it".

Zelensky said Tuesday that Ukraine was enjoying the most diplomatic and military backing since Russia's 2014 annexation of Crimea.

"Support for Ukraine is the biggest since 2014, unconditional and continues," he told Ukraine's parliament.

Tensions between Russia and the West over Ukraine have been building for weeks, with Washington accusing Moscow of preparing an imminent invasion of its pro-Western neighbour.

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 US-led military alliance's expansion in the ex-Soviet bloc.

The crisis has sparked a flurry of diplomacy -- both in person and through the exchange of documents -- with US Secretary Antony Blinken and Russian Foreign Minister Sergei Lavrov set for another phone call on Tuesday.

'Toughest sanctions ever'

The State Department said Monday it had received a letter from Moscow with its stance on Ukraine, but Russian officials said it was not an official response to previous US proposals.

Tuesday's phone call comes a day after fiery exchanges at the UN Security Council, where Washington's envoy said Russia was now also planning to send more than 30,000 troops near the Belarus-Ukraine border.

The US said it was telling families of its government employees in Belarus to leave the country.

Russia's envoy to the UN said Washington was "in hysterics".

"The discussions about a threat of war is provocative in and of itself. You are almost calling for this, you want it to happen," Vassily Nebenzia said.

Western leaders have repeatedly warned of "severe consequences" if Russia does invade, including wide-ranging and damaging economic sanctions.

Britain and the United States said Monday they were looking at targeting people in President Vladimir Putin's inner circle, including powerful business allies.

British Foreign Secretary Liz Truss told parliament that the government was putting through "the toughest sanctions regime against Russia we've ever had".

"Those in and around the Kremlin will have nowhere to hide," she said.

Hungary's Orban in Moscow

The Russian embassy in Washington said Moscow had every right to move troops on its own territory.

"We are not going to back away and stand at attention, listening to the threats of US sanctions," the embassy said on Facebook, adding that it is "Washington, not Moscow, that generates tensions."

The United States and Britain have been at the forefront in warning of an invasion and have sent new shipments of weapons to shore up the Ukrainian military.

Johnson and Zelensky will on Tuesday discuss the "ongoing Russian hostile activity", a British statement said, covering "the full range of strategic UK support to Ukraine".

Putin was meanwhile to host one of his few allies in the European Union and NATO, Hungarian Prime Minister Viktor Orban.

The Kremlin said the leaders will discuss trade and energy as well as "the current problems of ensuring European security," as Orban looks for an agreement to increase Hungary's gas imports from Russia.

He was travelling to Moscow in defiance of calls to cancel the trip from opposition parties, who said in a joint statement that it is "contrary to our national interests".

French leader Emmanuel Macron spoke to Putin on Monday for the second time in four days, with the Kremlin saying the two exchanged views on "issues related to providing Russia with long-term and legally binding security guarantees".

Ukraine has been battling Moscow-backed insurgencies in two separatist regions since 2014, with more than 13,000 people killed in the conflict.

Kyiv has welcomed recent Western support, but Zelensky has warned against creating "panic" with talk of an invasion at any moment.