Who is in Nato and what are Article 4 and Article 5?

Poland's Prime Minister Donald Tusk has said Polish and other Nato aircraft shot down a number of Russian drones which violated the country's airspace during strikes on neighbouring western Ukraine on the night of 9-10 September.

While Ukraine is not a Nato member, Poland is, and Tusk asked the alliance to investigate whether the territorial integrity, political independence or security of a member country had been threatened.

Then on 19 September, Estonia requested a consultation with other Nato members after three Russian MiG-31 fighter jets violated its airspace. Russia later denied any violation by the jets, saying they had flown over "neutral" Baltic waters.

At the Nato summit in June, the alliance's 32 members restated their "ironclad commitment to collective defence", known as Article 5, and agreed to boost defence spending after months of pressure from US President Donald Trump.

Nato - the North Atlantic Treaty Organization - was formed in Washington DC in 1949 by 12 countries.