This photo shows military equipment positioned in convoy at Lake Donuzlav in Crimea on February 15. AP

As Ukrainians waved flags in a show of defiance of a feared Russian invasion, the United States reported that Moscow had added as many as 7,000 troops to forces stationed along the tense border — a warning that contradicted Kremlin declarations that military units were being pulled back.

A Russian invasion of Ukraine did not materialise on Wednesday, as originally feared. But after a handful of positive signals from Moscow that eased tensions earlier in the week, the pendulum appeared to swing in the opposite direction again.

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Meanwhile, Russia's defence ministry published video on Thursday that it said showed troops and military equipment from its western military district's tank army returning to their permanent deployment bases after drills.

The ministry said tanks and armoured vehicles would travel around 1,000km (620 miles) by rail.

Western countries warned on Wednesday there was a growing Russian military presence at Ukraine's borders, contradicting Moscow's insistence of a pullback.

Russian Defence Minister Sergei Shoigu inspects the exercises of Navy in the Mediterranean Sea. File/Reuters

Western allies maintained that the threat of an attack was strong, with an estimated 150,000-plus Russian troops surrounding the country on three sides.

At the heart of the crisis are Russia's demands that the West keep Ukraine and other former Soviet nations out of NATO, halt weapons deployments near Russian borders and roll back forces from Eastern Europe. The US and its allies have roundly rejected those demands, but they offered to engage in talks with Russia on ways to bolster security in Europe.

Though Russia has said it is pulling back some troops, a senior US administration official said some forces arrived only recently and that there had been a marked increase in false claims by Russians that the Kremlin might use as pretext for an invasion. The official said those claims included reports of unmarked graves of civilians allegedly killed by Ukrainian forces, assertions that the US and Ukraine are developing biological or chemical weapons, and claims that the West is funneling in guerrillas to kill Ukrainians.

The official was not authorized to speak publicly about sensitive operations and spoke to The Associated Press on condition of anonymity. The official did not provide underlying evidence for the assertions.

US Secretary of State Antony Blinken speaks during a press conference. File photo

"We haven’t seen a pullback,” US Secretary of State Antony Blinken told ABC News. Russian President Vladimir Putin "can pull the trigger. He can pull it today. He can pull it tomorrow. He can pull it next week. The forces are there if he wants to renew aggression against Ukraine.”

Asked why Russians would claim to be withdrawing when government intelligence, commercial satellite photos and social media videos showed no evidence of that, State Department spokesman Ned Price said: "This is the Russian playbook, to paint a picture publicly … while they do the opposite.”

Maxar Technologies, a commercial satellite imagery company that has been monitoring the Russian buildup, reported that new photos show heightened Russian military activity near Ukraine, including the construction of a pontoon bridge in Belarus less than 6 kilometers (4 miles) from the Ukrainian border.

A satellite image shows a close up of a field hospital at the Osipovichi training area in Belarus. Reuters

NATO Secretary-General Jens Stoltenberg said the alliance also had not seen "any withdrawal of Russian forces,” as did some European governments.

Ukrainian President Volodymyr Zelenskyy similarly dismissed the Russian claims.

"What is this? Rotations, withdrawal, returning back again," he said on a visit to the southeastern city of Mariupol. "It’s too early to rejoice.”

The Ukrainian leader, who has repeatedly sought to project calm and strength during the crisis, declared Wednesday a day of "national unity” - a day that had been floated as a possibility for the start of an invasion.

Ukrainian President Volodymyr Zelenskyy attends a meeting. File photo

"We are united by a desire to happily live in peace,” Zelenskyy told the nation in an address. "We can defend our home only if we stay united.”

Across the country, Ukrainians of all ages waved flags in the streets and from apartment windows.

Hundreds unfolded a 200-metre (650-foot) flag at Kyiv’s Olympic Stadium, while another was draped in the center of a shopping mall in the capital.

In the government-controlled part of Ukraine's eastern region of Luhansk, where Russian-backed separatists have been fighting Ukrainian troops since 2014, residents stretched another huge flag across a street.

"This event, this number of people united around the Ukrainian flag will show that we stand for united Ukraine,” resident Olena Tkachova said.