Live updates: follow the latest news on Russia-Ukraine
US President Joe Biden said on Thursday every indication shows Russia is now planning to invade Ukraine, including signs Moscow is carrying out a "false flag operation" to justify it, after Ukrainian forces and pro-Moscow rebels traded fire.
Mr Biden's remarks came shortly after Ukraine and pro-Russian separatists exchanged fire across the front line, in what western officials described as a possible pretext created by Moscow to invade.
"We have reason to believe they are engaged in a false flag operation to have an excuse to go in. Every indication we have is they’re prepared to go into Ukraine and attack Ukraine," Mr Biden told reporters as he departed the White House.
A false flag operation is one in which an attack is staged or falsely attributed to a military force or foreign power and then used as a pretext for war.
Moscow, for its part, ejected the number two official from the US embassy last week and released a strongly worded letter to the US accusing it of ignoring its security demands.
Meanwhile at the UN, US Secretary of State Antony Blinken squared off against Russian diplomats at the Security Council. He said the US still sought a diplomatic resolution but was set to impose "severe measures" if Russia invades.
Speaking in New York, Mr Blinken explained how Russia may "manufacture a pretext" for invading Ukraine, adding that such a false flag operation may be "unfolding right now" in the tense border area.
It could be a "fabricated so-called terrorist bomb" attack on Russia, a mass grave discovery, a staged drone or chemical weapons strike, followed by Russian officials "theatrically" holding emergency talks and alleging "ethnic cleansing or a genocide" against the Russian-speaking population, Mr Blinken said.
He invited Russian Foreign Minister Sergey Lavrov to meet him in Europe next week to address Russian security concerns, and called for talks with Europeans and Nato to avert the crisis.
"The Russian government can announce today with no qualification, equivocation or deflection, that Russia will not invade Ukraine — stated clearly, stated plainly to the world and then demonstrated by sending your troops, your tanks, your planes back to their barracks and hangers and sending your diplomats to the negotiating table," Mr Blinken said.
Russian Deputy Foreign Minister Sergey Vershinin said Mr Blinken's military scenarios were regrettable and dangerous and said some Russian soldiers were already returning to home bases.
He blamed tension in eastern Ukraine on the "intense stepping up of drills of Nato, including alarmist propaganda that we cannot ignore" and accused Kiev of failing to deliver on a 2014 peace agreement.
Yasar Halit Cevik, the chief monitor for an Organisation for Security and Co-operation in Europe mission to eastern Ukraine, described about 500 explosions overnight on Wednesday along the front line but said they had slowed and “may seem to be easing”.
The UN's political chief Rosemary DiCarlo said tension was "running higher" in Ukraine than at any point since the last Russian incursion into the country in 2014 and called the deepening crisis "extremely dangerous".
She called for "maximum restraint" amid fresh shelling incidents in Eastern Ukraine.
"If verified, these must not be allowed to escalate further," Ms DiCarlo said.
"Ukraine’s sovereignty, independence and territorial integrity within its internationally recognised borders must be respected."
Lana Nusseibeh, the UN ambassador for the UAE, which holds a two-year seat on the council, called for "constructive dialogue" to stop tension escalating into a full-blown confrontation.
Before the council meeting, Linda Thomas-Greenfield, the US ambassador to the UN, said Russia was still moving towards an “imminent invasion” after amassing about 150,000 troops, tanks and other hardware along its frontier, in Moscow-friendly Belarus and Crimea.
Russia seeks assurances that Ukraine will never join Nato, among other guarantees. It has repeatedly denied planning to invade its neighbour, saying troop movements are routine and claiming to have pulled some forces back, while accusing western critics of “hysteria”.
The UN talks were planned before the current crisis and focused on the Minsk agreements on the eastern Donbas region of Ukraine, where Moscow-aligned secessionists have been battling Ukrainian government forces since 2014.
“Our goal is to convey the gravity of the situation,” Ms Thomas-Greenfield said.
“The evidence on the ground is that Russia is moving towards an imminent invasion. This is a crucial moment.”
Mr Blinken was set to attend a security conference in Munich this week, but detoured through New York to “emphasise the path towards de-escalation and to make it clear to the world that we are doing everything we can to prevent war”, added Ms Thomas-Greenfield.
The 15-nation council met against a backdrop of conflicting messages on Ukraine.
Moscow claims it is pulling back troops from its borderlands, while Washington said it is building up its readiness for combat.
Late on Thursday, the US Senate voted overwhelmingly for a resolution to show unwavering support for an independent Ukraine and “condemn” Russian military aggression toward its neighbour.
Agencies contributed to this report