According to Lufthansa, it is closely monitoring the situation in Ukraine and is considering stopping air traffic.

At the moment, however, there is no decision on this, the company announced on request.

The Dutch airline KLM announced on Saturday that it would discontinue its flight connections to Ukraine with immediate effect.

Amid the tensions, Russia said it had driven an American submarine out of its own territorial waters in the Pacific on Saturday.

The destroyer "Marshal Shaposhnikov" has tracked down a submarine in Russian waters near the Kuril Islands, the Defense Ministry said in Moscow on Saturday.

After the submarine ignored requests to heave to, the Russian destroyer "took appropriate action," it said without further explanation.

The submarine then left Russian waters “at top speed”.

At the time, exercises by the Russian Navy were planned near the island of Urup, which belongs to Russia.

The American military attaché was later summoned to the Defense Ministry in Moscow and given a protest note.

US denies incident

The US has rejected the account of Russia.

A spokesman for the US Navy for the Indo-Pacific region said on Saturday (local time) when asked by the dpa news agency: "The Russian claims that we are operating in their territorial waters are not true." He will not comment on the exact location of U.S. boats say, "but we fly, sail and operate safely in international waters," the spokesman said.

The Russian Defense Ministry had previously announced that a US submarine had been expelled from its own territorial waters in the Pacific.

The Virginia-class boat violated the Russian state border near the Kuril Islands on Saturday.

Exercises by the Russian Navy were planned there at the time.

Because the US crew initially ignored warnings, the Russian side used "appropriate means" to force the boat to turn back, it said.

Details were not initially given.

The American military attaché was later summoned to the Defense Ministry in Moscow to hand him a note of protest.

The incident happened against the background of high tensions between the United States and Russia over the Ukraine crisis.

According to Western sources, Russia has massed more than 100,000 soldiers on the Ukrainian border in recent weeks.

The troop build-up and Russian military exercises in neighboring Belarus and in the Black Sea are fueling fears in the West of an invasion of Ukraine.

Moscow rejects any attack plans and at the same time accuses Kiev and NATO of "provocations".

Diplomatic efforts to resolve the crisis at the highest level continued on Saturday.

US President Joe Biden spoke to Russian President Vladimir Putin on the phone.

The conversation lasted about an hour, the White House said afterwards in Washington.

French President Macron also spoke to Putin on Saturday and called on the Russian President to comply with the Minsk Agreement.

The American employees of the OSCE observer mission are meanwhile apparently withdrawing from Ukraine.

This was reported by an eyewitness from Donetsk.

It was said that they had driven out of town by car.

The OSCE initially did not comment on this.

A spokesman for the US State Department had previously called on US OSCE staff to leave the country in view of a possible threat of a Russian invasion: the corresponding travel advice for US citizens from last week also applies to them.