MOGADISHU, (Reuters) – A spokesman for Somalia’s al Shabaab rebels denied on Friday that the group was behind a suicide bombing at a medical graduation ceremony that killed at least 22 people, including three government ministers. But analysts pointed out that the bloodshed had been a PR disaster for the insurgents, and the U.N. envoy to the country said it was “outrageous” to suggest anyone else was to blame.

Doctors, students and their parents were among the dead at Mogadishu’s Shamo Hotel following Thursday’s attack, which was the worst in the failed Horn of Africa state for five months.

Suspicion had immediately fallen on the hardline al Shabaab group, which is battling the Western-backed government to impose its harsh interpretation of Islamic law across the country. “We declare that al Shabaab did not mastermind that explosion … we believe it is a plot by the government itself,” al Shabaab spokesman Sheikh Ali Mohamud Rage told reporters. “It is not in the nature of al Shabaab to target innocent people.”

Rage said serious political rifts had emerged between senior figures in President Sheikh Sharif Ahmed’s administration, which controls little more than a few strategic areas of the capital. “You know there is a power struggle … that has been going on a long time,” the insurgent spokesman said. “We know some so-called government officials left the scene of the explosion just minutes before the attack. That is why it is clear that they were behind the killing.”

Western security agencies say Somalia has become a safe haven for militants, including foreign jihadists, who are using it to plot attacks across the impoverished region and beyond.

The U.N special envoy to Somalia, Ahmedou Ould-Abdallah, said insurgents had been behind most of the recent high profile acts of violence to afflict the nation.

“I think it is outrageous to suggest now that behind this killing is not the same group that killed the security minister, that attacked AMISOM (peacekeepers), that has stoned women and children to death,” he told Reuters by telephone from Tokyo.

The United States accuses al Shabaab, the only Somali rebel group to have launched suicide attacks in the past, of being al Qaeda’s proxy in the drought-ravaged country.

Kamal Dahir, a Somali political analyst based in Nairobi, said al Shabaab has issued its denial very late. “I think that, after the explosion, al Shabaab commanders were sitting for hours to finalise what to do … We are all convinced that the nature of the bombing and tactics used were similar to the group’s previous attacks,” Dahir told Reuters. “They realised it would turn the public against them, so denied being responsible to keep the little support they have.”

In June, al Shabaab said it was behind a suicide bombing in Baladwayne town that killed Somalia’s security minister and at least 30 other people. Then in September it struck the heart of the African Union’s main military base in Mogadishu with twin suicide car bombs, killing 17 AMISOM peacekeepers.

Witnesses said Thursday’s bombing was carried out by a man disguised as a veiled woman. He entered the ceremony, packed with graduates of Benadir University’s medical school and their relatives before approaching the podium and blowing himself up.

Ahmed’s government had been preparing for a new offensive against the rebels in recent weeks, and Thursday’s carnage looked sure to heighten its frustration over delayed pledges of military and financial support from Western donors.

Fighting has killed at least 19,000 Somali civilians since the start of 2007 and driven another 1.5 million from their homes. The chaos has also spilled offshore, where heavily armed Somali pirates have made tens of millions of dollars in ransoms.

Somali Prime Minister Omar Abdirashid Ali Sharmarke denounced Thursday’s attack as “beneath contempt”, but he said it would not deter the government from fighting insurgents. “The loss of our ministers is disastrous, but it is an outrage to target the graduation of medical students and kill those whose only aim in life was to help those most in need in our stricken country,” he said in a statement.