Ryanair, one of the largest foreign airlines in Ukraine, said today it had a duty to fly passengers in and out of the country as long as a Russian invasion did not materialise.

"Is it our duty and obligation to support the people of Ukraine as long as there is no war or missiles flying there," Ryanair's group chief executive Michael O'Leary told a news conference in Lisbon.

Michael O'Leary said he saw no reason to halt flights unless European authorities say it is not safe to fly to Ukraine.

"It is important not to panic," he said.

"People need to get home and people want to leave and fly abroad to the EU - airlines have to provide that service," he stated.

Ryanair said last month it could base up to 20 planes in Ukraine over the next few years if Russia does not invade, and announced plans to fly 230 flights weekly flights from three Ukrainian airports this summer.

Asked if the plan still stood, O'Leary said the airline was reviewing the situation in Ukraine daily but that the country remained a "huge potential market as long as there is no Soviet invasion".

Several airlines have either halted flights to Ukraine or are considering doing so.

Ukrainian authorities have said they are keeping their airspace open.

NATO accused Russia today of sending more troops to a massive military build-up around Ukraine, even as Moscow said that it was withdrawing forces and was open to diplomacy.