By Reuters

LONDON - The leaders of the United States and Britain pledged on Tuesday to keep up the pressure on Libyan leader Muammar Gaddafi until he fully complied with United Nations resolutions.

Responding to the revolts sweeping the Arab world, President Barack Obama and Prime Minister David Cameron said in a joint article in The Times newspaper that they would not stand by as protesters' aspirations were "crushed in a hail of bombs, bullets and mortar fire."

"We are reluctant to use force but when our interests and values come together we know that we have a responsibility to act," they wrote.

"This is why we mobilised the international community to protect the Libyan people from Colonel Gaddafi's regime.

"We have degraded his war machine and prevented a humanitarian catastrophe. And we will continue to enforce the U.N. resolutions with our allies until they are completely complied with," the two leaders said. Obama arrived in Britain on Monday night, his second stop on a four-nation European trip.

NATO countries, led by Britain, France and the United States, have enforced a U.N.-authorised no-fly zone over Libya, inflicting severe damage on Gaddafi's military as he tries to stave off a revolt.

U.N. Security Council 1973, establishing the no-fly zone, calls for a ceasefire, an end to attacks on civilians, respect for human rights and efforts to meet Libyans' aspirations.