Britain on Friday intensified calls for its citizens to immediately leave Yemen and warned of an \"extremely serious escalation of violence\" as the country teetered on the brink of civil war. Foreign Secretary William Hague also said Britain would be unable to evacuate its nationals due to the deteriorating security situation, in a strongly worded statement released shortly after Yemen\'s president was lightly injured in an attack. \"Today\'s extremely serious escalation of violence in Yemen underscores our clear message that British nationals in Yemen should leave immediately while commercial flights are still operating,\" said Hague. \"Given that we cannot expect forewarning of any airport closures, British nationals should not wait to leave. \"We have already said that in such a difficult security situation it is extremely unlikely the British government will be able to evacuate British nationals. \"In light of the latest escalations, I cannot restate this strongly enough.\" \"You should not plan for nor expect the British government to be in a position to assist you to reach safety,\" he added. Britain first advised its nationals to leave Yemen on March 12 and has repeated the call several times since, but Friday\'s statement was the most strongly worded amid a dramatic escalation of unrest in the crisis-hit country. It came shortly after Yemen\'s embattled President Ali Abdullah Saleh was wounded along with his premier and other officials when shells fired by dissident tribesmen smashed into the presidential palace. In an audio message on state television after the attack, Saleh said: \"I am well, in good health.\" Saleh, who has been in power in Sanaa since 1978, has faced nationwide protests against his rule for the past four months. More than 200 demonstrators have been killed since the protests erupted, according to an AFP tally based on reports from medics.