NATO chief Jens Stoltenberg said Tuesday the alliance was expelling seven Russian diplomats and denying accreditation to three more as part of international measures over the poisoning of a former spy in Britain.

"I have today withdrawn the accreditation of seven staff of the Russian mission to NATO. I will also deny the pending accreditation request for three others," Stoltenberg told a news conference at NATO headquarters in Brussels.

"This will send a clear message to Russia that there are costs and consequences for their unacceptable pattern of behaviour."