The group of major Western and regional powers also urged "all relevant parties" to make efforts "for the formation of an interim government to ensure a smooth and peaceful transition of power," said the statement, to be formally issued at the end of a meeting in Istanbul later Friday.

Together with France, Britain has taken a lead role in the NATO mission to protect Libyan civilians from attack by Gaddafi's forces and enforce a no-fly zone.

"The people of Benghazi are there and the city was not stormed because of the NATO action. The same applies to Misrata, equally many people in the Western mountains have benefited from that," Hague said.

"Of course, we all want to follow it through to the real success of the political settlement in Libya that requires Colonel Gaddafi to go," Mr Hague said. "Military action will intensify."

NATO Secretary-General Anders Fogh Rasmussen called on Thursday for members of the alliance to provide more aircraft to bomb Gaddafi's forces.

Britain said on Friday it would send four more Tornado warplanes for the NATO mission in Libya, in addition to the 12 already deployed to support the more modern Eurofighter Typhoons.