Cardiff, Sep 5 (IANS): NATO leaders Friday agreed on a readiness action plan to strengthen the bloc's collective defence and deal with any challenge, NATO Secretary General Anders Fogh Rasmussen said.

The NATO allies face a security environment which is more unpredictable than ever, including Russia's "aggression" against Ukraine, violent extremism in the Middle East and instability in North Africa, Xinhua quoted Rasmussen as saying at a press conference on the second day of the NATO summit in the Welsh city of Newport.

NATO leaders agreed to maintain a "continuous presence" and activity in the air, on land and at sea in the eastern part of the alliance, on a rotational basis, the NATO chief said.

"In these turbulent times, NATO must be prepared to undertake the full range of missions and to defend allies against the full range of threats," he added.

They also agreed to create a spearhead unit within the NATO Response Force which would be a very high readiness force able to deploy at very short notice.

Britain has pledged to contribute 1,000 military personnel to the NATO force that could be deployed anywhere in two-five days, BBC reported.

Speaking at the summit, British Prime Minister David Cameron said the "multinational spearhead force" would be part of a reformed NATO response force with headquarters in Poland.

Downing Street later said the force would comprise 4,000 troops, be led by a British General and would be ready for deployment by the end of 2015.