EU leaders have demanded that Muammar Gaddafi give up power immediately, and warned that "all necessary options" were being considered to protect Libya's civilian population.
A statement at the end of an emergency summit in Brussels did not explicitly mention imposing a no-fly zone, despite Anglo-French pressure in a letter to the other leaders from David Cameron and French President Nicolas Sarkozy.
Before returning to London, the Prime Minister denied being frustrated by the decision, insisting: "I think 'all necessary options" is pretty tough when it comes to these dreadful events that are happening in Libya.
"I think it is progress but sometimes progress can take some time when you have got 27 conversations around the table," he said.
But he acknowledged that "words are not enough" and insisted the international community had to be ready to act if the situation deteriorates.
He said it appeared that the situation of the opposition forces seeking to oust Gaddafi was getting "worse, not better", and warned that "horrors" may have been committed by the dictator's forces away from the eyes of the world.
Mr Cameron added: "Today, EU leaders were united, categorical and crystal-clear - Gaddafi must go. This man is brutalising his own people and we can't stand by while that happens.
"Clearly every day he goes on brutalising his own people is a bad day for humanity, is a bad day for people in Libya, and we should be standing with those people who want a better future for Libya, and that future cannot include Colonel Gaddafi."
The Prime Minister continued: "We also agreed to tighten the net on him and his henchmen, so we have strengthened the financial sanctions on the regime, adding the Libyan Central Bank and Libyan Investment Authority to the EU asset-freezing list. In doing so, the UK has frozen a total of £12 billion of Libyan assets."
The summit declaration condemned the "violent repression" against civilians.