Somalia's prime minister has resigned amid a power struggle with the country's president. President Sheikh Sharif Ahmed's UN-backed government is battling Islamist group al-Shabab for control of the capital, Mogadishu. Correspondents say his squabbling with the prime minister has hampered the fight against the insurgents. Omar Abdirashid Ali Sharmarke has been under intense pressure to resign for some months. He said he had been "unable to work with the president". The BBC's Mohammed Olad Hassan in Somalia says little is likely to change on the ground as a result of the resignation. However, he said it could come as a blow to the insurgents who were happy for the political turmoil to continue. Islamist militants now control most of southern and central Somalia and the government, backed by African Union peacekeepers, runs only a few parts of the capital. Somalia has been wracked by conflict ever since President Siad Barre was ousted in 1991. But insurgents from the al-Qaeda linked al-Shabab group have recently intensified their campaign. Earlier this month suicide bombers attacked Mogadishu airport, killing two African Union peacekeepers and a number of civilians. 'Save the nation' Mr Sharmarke said he had decided to step down in the national interest. "After seeing that the political turmoil between me and the president has caused security vulnerability, I have decided to resign to save the nation and give a chance to others," he told reporters. President Ahmed thanked him for his "courageous decision". The two leaders disagreed over a new draft constitution. A vote of confidence in the prime minister had been due to take place at the weekend but was postponed. In May, Mr Sharmarke rejected as unconstitutional a vote by parliament calling for him to stand down. President Ahmed is now expected to name a new prime minister and the current cabinet will lose their posts. Our correspondent says his position at the head of the government will have been boosted by the prime minister's resignation. Credit: BBC