"We have decided to restrict the access of Belarusian personnel to the NATO headquarters, based on our assessment of six security measures at the headquarters," Secretary General Jens Stoltenberg said.

According to a NATO official speaking on condition of anonymity, five Belarus envoys including an ambassador would no longer have privileged access to meetings at the building.

"They can still enter, but only as visitors with a day pass and an escort," he said, describing the protocol for non-official visitors to the headquarters.

Belarus is not a NATO member but has had a diplomatic mission to the Alliance since 1998. It is part of a Euro-Atlantic Partnership Council which oversees cooperation between allies and non-members in crisis management and arms control.

Monday's diplomatic move is largely symbolic but reflects outrage felt in Western capitals at Belarusian President Alexander Lukashenko's decision to divert a Ryanair passenger jet to arrest an exiled dissident.

"We strongly condemn the forced landing of civilian aircraft in Minsk. We call on an independent international investigation," Stoltenberg said, welcoming sanctions imposed on Minsk by some EU and NATO member states.

"And we also clearly state that this is not only something which is violating national norms and rules, but also a direct attack on the freedom of expression on the free and independent press."

- Russia tensions -

The move comes at a low point in relations between the NATO allies and Russia, Belarus's chief sponsor which has also seen measures taken against Russian diplomats.

NATO headquarters is preparing to host, on June 14, a summit of member state leaders -- including for the first time US President Joe Biden -- and Russia will be high on the agenda.

"What we see is a pattern of Russian behaviour where Russia over the last years has invested heavily in new modern military capabilities from conventional to nuclear weapon systems," Stoltenberg said.

"Not only that, but that Russia has been willing to use military force against neighbours in Georgia and Ukraine, continuing to destabilise Donbas and eastern Ukraine and illegally annexing Crimea.

"And then we've seen more Russian military presence in the high north, in the Barents Sea and in the Baltic Sea, Kaliningrad, the Black Sea, and also down to the Mediterranean and Middle East.

"And this is one of the main reasons why NATO over the last years have increased the readiness of forces."