MILAN (AP) — Tensions spiked anew over Ukraine on Thursday amid conflicting claims over whether Russia had drawn down troops it has been massing for weeks around Ukraine, escalating hostilities in Ukraine's separatist-controlled east and intensified diplomacy.

U.S. President Joe Biden warned there is still a "very high" risk of a Russian invasion within "several days." And, in what the United States described as an unprovoked move, Russia expelled a senior U.S. diplomat in Moscow.

A day after Moscow said it was returning troops to bases, the NATO allies said Russia is actually building up forces near Ukraine. At the same time, Ukraine and the rebels in its eastern territory accused each other Thursday of intensive shelling along the line of contact in Donetsk and Luhansk.

U.S. Secretary of State Antony Blinken, on his way to an annual security conference in Germany, was rerouted to the United Nations to promote a diplomatic solution. 

Here's a look at what is happening where and why:

What we do and don't know on the ground

Russia says it moved its troops away from the conflict zone. But NATO allies say constant movements mask their true intentions. 

NATO allies accuse Russia of misleading the world by saying some 7,000 troops were being returned to bases, but instead moving in thousands of new ones.

Maxar Technologies, a commercial satellite imagery company that has been monitoring the Russian buildup, reported continued heightened military activity near Ukraine, including a new pontoon bridge and a new field hospital in Belarus.

Russia has massed an estimated 150,000-plus troops on three sides of Ukraine in recent weeks, but denies it is plotting an invasion.

The NATO Secretary-General Jens Stoltenberg said Russia has "enough troops, enough capabilities, to launch a full-fledged invasion of Ukraine with very little or no warning time, and that is what makes the situation so dangerous."

"The fact that you're putting a battle tank on a train and moving it in some direction doesn't prove a withdrawal of troops," he said. "It has to be a meaningful withdrawal, a meaningful de-escalation."

Shelling along the border in Ukraine's separatist-controlled east also has fueled speculation that Russia might be creating a pretext to invade Ukraine. A sharp increase in skirmishes in recent days raised that specter.

In the latest incident, separatist authorities in the Luhansk region reported in an increase in Ukrainian shelling, calling it a "large-scale provocation." The Ukraine military said it had not fired back after its forces were shelled. They said the shells also hit a kindergarten, wounding two civilians.

The diplomatic front

As EU and NATO officials sent out assurances that there's still room for diplomacy, Russia expelled the second-highest U.S. diplomat in Moscow.

Washington said the expulsion of U.S. deputy chief of mission Bart Gorman was an unprovoked escalation in tensions. "We are considering our response," the State Department said.

How does the United States see a Russian invasion unfolding?

U.S. Secretary of State Antony Blinken went before the U.N. Security Council Thursday to sketch out how Washington contends a Russian attack would unfold, revealing conclusions of U.S. intelligence in a strategy the United States and Britain have hoped will pre-empt any Russian invasion planning. The United States has declined to reveal most of the evidence underlying its claims on Russia's planning.

A sudden, seemingly violent event staged by Russia to justify invasion would kick it off, Blinken told U.N. diplomats. "We don't know exactly" the pretext, he said – a "so-called terrorist bombing" inside Russia, a staged drone strike, "a fake, even a real attack … using chemical weapons."

The military attack itself would open with cyber attacks and missiles and bombs dropping across Ukraine, America's top diplomat said. Painting the U.S. picture further, Blinken described the entry of Russian troops, advancing on Kyiv, a city of nearly 3 million people, and other "key targets that have already been identified and mapped out."

U.S. intelligence indicated Russia would target "specific groups" of Ukrainians, Blinken told the U.N., again without giving details. In an implicit nod to Secretary of State Colin Powell's appearance before the Security Council in 2003, when Powell cited false U.S. intelligence to justify the U.S. invasion of Iraq, Blinken added: "Let me be clear. I am here today not to start a war, but to prevent one."

What is Moscow saying?

Russia on Thursday reaffirmed its demand for the U.S. and its allies to keep Ukraine out of NATO but held the door open for talks on a range of security issues.

The Russian Foreign Ministry handed over its formal reply to the U.S. and NATO security proposals and later published it on its website.

The document again denies Western claims that Russia has an intention to invade Ukraine, but repeats that NATO's expansion to Ukraine and other ex-Soviet nations represents a "red line" for Russia.

It says the U.S. and its allies have stonewalled Russia's demands to keep Ukraine and other ex-Soviet nations out of NATO, halt weapons deployments there and roll back alliance forces from Eastern Europe. The paper reaffirmed that Moscow would continue pressing those demands and could take unspecified "military-technical measures" if the West continues to ignore them.

At the same time, it said Russia is ready to discuss measures to enhance security in Europe by negotiating limits on missile deployments, restrictions on patrol flights by strategic bombers and other confidence-building steps provided they are discussed in conjunction with Moscow's main proposals.

How are Ukrainians reacting?

Ukrainian President Volodymyr Zelenskyy suggested that NATO membership is a distant dream for his country, blaming both opposition from Russia and from some unidentified NATO members.

"It is not the Ukrainian people who choose when we will be (in NATO), because it depends not only on us: 30 countries must unanimously agree on this decision," Zelenskyy said.

What is NATO doing

Stoltenberg opened a meeting of the alliance's defense ministers in Brussels. He said Russia has "yet again demonstrated its disregard for the principles underpinning European security, and its ability and willingness to threaten the use of force in pursuit of its objectives."

U.S. Defense Secretary Lloyd Austin told reporters in Brussels that Russia is moving troops close to the border, flying in more combat and support aircraft. 

"We see them sharpen their readiness in the Black Sea. We even see them stocking up their blood supplies," he said. "You don't do these sort of things for no reason, and you certainly don't do them if you're getting ready to pack up and go home."

Meanwhile, the alliance is bolstering its eastern regions.

The U.S. has started to deploy 5,000 troops to Poland and Romania. Britain is sending hundreds of soldiers to Poland and offering more warships and planes. It also is doubling the number of personnel in Estonia and sending tanks and armored fighting vehicles. Germany, the Netherlands and Norway are sending additional troops to Lithuania.