KYIV, Ukraine (AP) -- More NATO troops headed to Eastern Europe and some nations worked to move their citizens and diplomats out of Ukraine on Monday, as Germany's chancellor made a last-ditch attempt to head off a feared Russian invasion that some warn could be just days away.
With the world already on high alert, U.K. Armed Forces Minister James Heappey said a Russian attack could "effectively now happen with no notice." That follows a warning from U.S. officials that an invasion could come this week -- leading to a flurry of diplomacy but also deterrence measures.
German Chancellor Olaf Scholz visited Ukraine on Monday and plans to continue on to Moscow, where he will try to persuade Russian President Vladimir Putin to stand down. Moscow denies it has any plans to invade but has massed well over 130,000 troops near Ukraine and, in the U.S. view, has built up enough firepower to launch an attack on short notice.
"We are experiencing a very, very serious threat to peace in Europe," Scholz wrote on Twitter as he arrived in the Ukrainian capital of Kyiv. "From Moscow, we urgently expect signals of de-escalation."
With concerns rising that war could be imminent, German's military said the first of some 350 extra troops it is sending to bolster NATO forces in Lithuania were en route Monday. Six howitzer guns were also being loaded onto trucks for transport to the alliance's eastern flank.
Meanwhile, Lithuania moved diplomats' families and some nonessential diplomatic workers out of Ukraine; the U.S. is already pulling most of its staff from the embassy in Kyiv. And the Greek Foreign Ministry joined several Western nations in urging its citizens to leave the country.
The moves were the latest preparations for a possible war. On Sunday, some airlines canceled flights to Kyiv and troops there unloaded fresh shipments of weapons from NATO members. Ukraine's air traffic safety agency declared the airspace over the Black Sea to be a "zone of potential danger" because of Russian naval drills and recommended that planes avoid flying over the sea Feb. 14-19.
The U.S. and its NATO allies have repeatedly warned that Russia will pay a high price for any invasion -- but they have sometimes struggled to present a united front. Scholz's government, in particular, has been criticized for refusing to supply lethal weapons to Ukraine or to spell out which sanctions it would support against Russia, raising questions about Berlin's resolve to stand up to Moscow.
So far, NATO's warnings appear to have had little effect: Russia has only beefed up troops and weapons in the region and launched massive drills in its ally Belarus, which also neighbors Ukraine. The West fears that the drills, which will run through Sunday, could be used by Moscow as a cover for an invasion from the north.
Russia has repeatedly brushed off Ukrainian and Western concerns about the military buildup, saying it has the right to deploy forces wherever needed on its territory.
Moscow wants guarantees from the West that NATO won't allow Ukraine and other former Soviet countries to join as members, and that the alliance will halt weapons deployments to Ukraine and roll back its forces from Eastern Europe. The U.S. and NATO have flatly rejected those demands.
Some observers expect Moscow to eventually accept a compromise that would help avoid hostilities and allow all sides to save face. While NATO refuses to shut the door to Ukraine, the alliance also has no intention of embracing it or any other ex-Soviet nation anytime soon. Some experts have floated ideas such as a moratorium on NATO expansion or a neutral status for Ukraine to defuse the tensions.
Ukraine's ambassador to the U.K., Vadym Prystaiko, seemed to suggest just such a middle path, telling the BBC on Sunday that the country could abandon its goal of joining NATO -- an objective that is written into its constitution -- if it would avert war with Russia.
"We might -- especially being threatened like that, blackmailed by that, and pushed to it," Prystaiko told BBC Radio 5.
On Monday, however, Prystaiko appeared to back away from that, saying that "to avoid war we are ready for many concessions ... but it has nothing to do with NATO, which is enshrined in the constitution."
Asked about Prystaiko's comment, Kremlin spokesman Dmitry Peskov said that Russia would welcome such a move but noted the quick repudation of it by the Ukrainian Foreign Ministry.
Ukrainian parliament speaker Ruslan Stefanchuk also emphasized that there was no talk about revising the constitutional provision that refers to NATO membership, and some lawmakers called for Prystaiko's dismissal.
Meanwhile, Ukrainian Foreign Minister Dmytro Kuleba said Sunday that Kyiv requested a meeting of the Organization for Security and Cooperation in Europe in the next 48 hours to discuss the Russian deployments near the country's borders.
Poland, which currently chairs the OSCE, said the meeting is scheduled for Tuesday -- but it is unlikely to defuse tensions.
With the region on edge, the Russian Defense Ministry summoned the U.S. Embassy's military attache on Saturday to protest what it said was a U.S. submarine in Russian waters near the Kuril Islands in the Pacific. The Russian military said the submarine initially ignored orders to leave, but left after the navy used unspecified "appropriate means." The U.S. has denied that its ship ever entered Russian waters.
Asked by lawmakers Monday if the military could strike foreign warships that enter Russian waters, deputy chief of the Russian military's General Staff Stanislav Gadzhimagomedov said the military stands ready for it, but added that such decisions are only made on the highest level.
High-level diplomacy has also continued -- but with little results so far. In an hourlong Saturday call with Putin, U.S. President Joe Biden said that invading Ukraine would cause "widespread human suffering" and that the West was committed to ending the crisis but "equally prepared for other scenarios," the White House said.
Biden also spoke to Ukrainian President Volodymyr Zelenskyy for about an hour Sunday, agreeing to keep pushing both deterrence and diplomacy to try to stave off a Russian offensive.
As he has before, Zelenskyy sought to play down the idea that a conflict was imminent, noting that Kyiv and other cities of Ukraine "are safe and under reliable protection."
His office's readout of the call also quoted him suggesting that a quick Biden visit would help deescalate the situation -- a possibility was not mentioned in the White House summary of the call.
Russia and Ukraine have been locked in a bitter conflict since 2014, when Ukraine's Kremlin-friendly leader was driven from office by a popular uprising. Moscow responded by annexing the Crimean Peninsula and then backing a separatist insurgency in eastern Ukraine, where fighting has killed over 14,000 people.
A 2015 peace deal brokered by France and Germany helped halt large-scale battles, but regular skirmishes have continued, and efforts to reach a political settlement have stalled.