A summit might offer a possible path out of Europe's biggest military crisis in decades.

However, both Washington and Moscow played down hopes of a breakthrough, and satellite imagery appeared to show Russian deployments closer to Ukraine's border than before.

Moscow denies planning any attack but has demanded security guarantees including a promise that Ukraine would never join NATO.

Nerves were further frayed when the Belarusian defence ministry announced on Sunday that Russia would extend military drills in Belarus that had been due to end.

Kremlin spokesperson Dmitry Peskov told reporters that a phone call or meeting between Putin and Biden could be set up at any time, but there were no concrete plans yet for a summit.

Tensions were growing, but diplomatic contacts were active and a meeting of foreign ministers was possible this week.

He also said Putin would imminently address an extraordinary session of Russia's Security Council.

The White House said in a statement that Biden had accepted the meeting "in principle" but only "if an invasion hasn't happened".

"We are always ready for diplomacy," White House Press Secretary Jen Psaki said. "We are also ready to impose swift and severe consequences, should Russia instead choose war."

Western countries are preparing sanctions they say would be far-reaching against Russian companies and individuals in the event that Russia invades, including steps to bar US financial institutions from processing transactions for major Russian banks, people familiar with the matter said.

EU foreign policy chief Josep Borrell said he still saw room for diplomacy, but would convene an extraordinary EU meeting to agree sanctions "when the moment comes".

French President Emmanuel Macron's office said in a statement that he had pitched a summit on "security and strategic stability in Europe" to both leaders.

Macron's office and the White House said the substance of the plan would be worked out by US Secretary of State Antony Blinken and Russian Foreign Minister Sergei Lavrov during a meeting planned for February 24.

US-based satellite imagery company Maxar on Sunday reported multiple new deployments of Russian military units in forests, farms, and industrial areas as little as 15km from the border with Ukraine.

Blinken said the extension of the exercises in Belarus made him more worried that Russia was on the brink of an attack.

In a letter to UN human rights chief Michelle Bachelet seen by Reuters, the United States said an invasion "could create a human rights catastrophe" and might include rounding up and killing opponents.

Washington has also repeatedly raised concerns that Russia could manufacture a pretext for an invasion with a so-called false flag attack designed to look as if it had been carried out by Ukraine. Russia accuses the West of hysteria.

Sporadic shelling across the line dividing Ukrainian government forces and pro-Russian separatists in the east has intensified since Thursday, with both sides trading blame.

The separatists have been busing out civilians, accusing Kyiv of planning an attack. Ukraine and the West view the rebels as Russian proxies, escalating to provide Moscow with a justification to invade.

Sounds of fighting were heard again on Monday, including a blast in the centre of the separatist-held city of Donetsk. The cause could not be determined.

Russia's FSB intelligence service said a shell fired from Ukrainian territory had hit a Russian border guard post in the city of Rostov, but that no one had been hurt.

The rebels said two civilians were killed in shelling by Kyiv government forces, Russia's RIA news agency said. Russian media reported 61,000 evacuees from east Ukraine had crossed into Russia.

Washington says accusations that Kyiv would intentionally escalate the conflict are absurd at a time when Russia has massed troops on the border.

Australian Associated Press