A flight from Madeira, Portugal to Kyiv was barred from entering Ukraine on Sunday due to the crisis with Russia and was forced to land in Moldova, its operator said.

Ukraine's low-cost carrier SkyUp said the flight's permission to cross into Ukraine was removed at the last minute by the plane's Irish leasing company.

"Despite the airline's best efforts and the Ukrainian government's readiness to speak to the leasing company, the plane's owner categorically denied (permission to enter Ukraine) once the plane was already on its way," SkyUp said a statement.

SkyUp said it was trying to arrange a transfer for the flight's 175 passengers from the Moldovan capital Chisinau to Kyiv.

The Ukrainian Foreign Ministry said it was helping arrange buses for the diverted passengers, according to Agence France-Presse (AFP).

The incident occurred a day after Dutch carrier KLM suspended flights into Ukraine until further notice due to mounting fears that Russia was preparing an invasion of the former Soviet state. KLM, part of Air France, said it would stop flying to Ukraine immediately, news agency ANP reported.

A growing group of European and other nations are urging their citizens to leave Ukraine. The United States Embassy in Kyiv on Saturday ordered the evacuation of non-emergency staff.

Industry analysts believe other international airlines may also ban flights into Ukraine soon because of the growing cost to travel insurers.

The international travel industry is still jittery following the crash of Malaysia Airlines flight MH17, which was shot down near a conflict zone in eastern Ukraine in July 2014, killing all 298 people on board.

Meanwhile, a senior Ukrainian official said on Sunday that Ukraine sees no point closing its airspace amid an escalation with Russia.

"The most important point is that Ukraine itself sees no point in closing the sky. This is nonsense. And, in my opinion, it somewhat resembles a kind of partial blockade," said Mykhailo Podolyak, an adviser to the Ukrainian president's chief of staff. "If particular air carriers decide to reconfigure the flight schedule, this certainly has nothing to do with the decisions or policies of our state," he told Reuters.

Washington said the Russian military, which has more than 100,000 troops massed near Ukraine, could invade at any moment. Moscow denies having any such plan and has described such warnings as "hysteria."