(Reuters) — Ukraine’s biggest airline said Monday that its insurers have terminated cover for at least some of its aircraft on flights in Ukrainian airspace, as Russia masses a huge military force on the country's border.

Ukraine International Airlines said it had received the notification from its insurers as experts said more airlines were expected to avoid Ukraine's airspace following a U.S. warning that Moscow could invade its neighbor any time.

“UIA is making efforts and is constantly negotiating with insurance companies, the ministry of infrastructure of Ukraine, the state aviation administration, and hopes for mutual understanding,” the Ukrainian airline said in a statement.

Earlier, Dutch airline KLM said it would halt flights to Ukraine and through its airspace, while Germany's Lufthansa said it was considering a suspension.

British Airways flights between London and Asia on Monday appeared to be avoiding the airspace, according to Reuters monitoring of flight tracking service FlightRadar24.

The same website showed tracks of a U.S. Air Force Global Hawk intelligence-gathering military drone sweeping back and forth across the country from west to east on Monday.

UIA said that, at the request of aircraft leasing firms, five Boeing 737-800 had been flown to Spain, while two other planes had been sent to Serbia for scheduled maintenance. UIA has 25 planes in its fleet, according to its website.

Leasing companies control the majority of the world's fleet.

UIA said it would report any further changes and asked passengers to closely follow its website for any news on the airline and its schedule.

A BA pilot on Sunday said on Twitter that there was a longer flight time for a freighter service from London to Bangkok because of “current geopolitics.” BA did not respond immediately to a request for comment.

“My guess would be that Ukraine will become unavailable pretty soon if what we've seen over the weekend crystallizes into a couple of more carriers actually pulling the pin,” said Mark Zee, founder of flight operations advisory firm OPSGROUP.

“I don't think it will be government advice that's doing it so much as it will be insurance-based unavailability or carriers looking at other carriers. So if you have KLM, Lufthansa and British Airways, for example, deciding not to overfly Ukraine at all, we're almost back into an MH17 scenario,” he said.

Malaysia Airlines flight MH17 was shot down over eastern Ukraine in 2014, killing all 298 people on board, two-thirds of them Dutch citizens. Some airlines were already avoiding the airspace after earlier shoot-downs of military aircraft.

Mr. Zee said that the avoidance of Ukraine's airspace would have the biggest routing impact on airlines from neighboring countries but that it was not expected to add much to the cost of long-haul flights.

Ukrainian carrier SkyUp said it had to divert a flight from Portugal to Ukraine on Saturday after the plane's owner barred it from entering Ukrainian airspace.

News agency Interfax Ukraine said Ukrainian insurance companies had received a notification from reinsurers that airlines were not covered for war risks.

Ukraine Prime Minister Denys Shmygal said Sunday that the government had allocated 16.6 billion hryvnia ($591.98 million) to ensure flight safety for insurance and leasing companies to guarantee the continuation of flights through its airspace.