David Cameron has said Nato and its members "will not be intimidated" by Russia in the wake of its actions in Ukraine.
Speaking ahead of the Nato summit in south Wales next month the prime minister said the gathering marked a "pivotal" moment in Nato's history.
He said: "In 2014, the world is more unpredictable than ever. To the East, Russia has ripped up the rulebook with its illegal annexation of Crimea and aggressive destabilisation of Ukraine.
"To the South, an arc of instability spreads from North Africa and the Sahel, to Syria, Iraq and the wider Middle East.
"So we must use the Summit to agree how Nato should adapt to respond to and deter such threats; and to ensure the continued collective defence of all its members."
The summit near Newport is the first in the UK since Margaret Thatcher hosted the alliance in 1990, as the Cold War was ending and Nato started to chart a course for a new relationship with Russia.