U.S. Secretary of State Antony Blinken on Thursday warned that Russian forces are preparing to launch an attack against Ukraine in "coming days," adding that Russia's invasion plans include capital Kyiv.

Russia plans to manufacture a pretext for an attack on its neighbor that could include a fake or real assault using chemical weapons, he also underlined.

Blinken told a U.N. Security Council meeting on Ukraine that he had sent a letter to Russian Foreign Minister Sergei Lavrov earlier on Thursday proposing a meeting in person in Europe next week, as he called on Russia to state clearly and plainly during the meeting that it would not invade Ukraine.

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 materialize 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.

Western allies maintained that the threat of an attack was strong, with an estimated 150,000 or more 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 U.S. 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 U.S. 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 a pretext for an invasion. The official said those claims included reports of unmarked graves of civilians allegedly killed by Ukrainian forces, assertions that the U.S. and Ukraine are developing biological or chemical weapons, and claims that the West is funneling in guerrillas to kill Ukrainians.