KAMPALA, July 26 (Reuters) - The United States added its voice on Monday to growing calls at an African summit for more troops to tackle Somalia's Islamist rebels, who this month killed 76 people in suicide attacks in Uganda.

Delegates at the African Union (AU) summit, being held in Kampala close to the site of the bombings, are debating the mandate of 6,300 AU peacekeepers in Somalia, which are barely managing to keep the country's besieged government in power.

Sources at the meeting told Reuters a cap of 8,100 on troop levels would likely be lifted. A more contentious possibility was that the force, known as AMISOM, be given permission to go after the rebels. It can now fight only when attacked.

"There is no doubt there is a need for more troops on the ground," Assistant Secretary of State Johnnie Carson told reporters on the sidelines of the meeting.

AMISOM is made of up of Ugandan and Burundian troops. Its alleged killing of civilians with indiscriminate shelling was the reason given by the al Qaeda-linked al Shabaab rebels for their bombings of crowds in Kampala watching the World Cup soccer final on television.

"We in Washington have committed ourselves to supplying the additional troops on the ground in the same fashion that we have supplied the existing Burundian and Ugandan troops," Carson said, referring to U.S. financial and technical help.

"SOURCE OF TERRORISM"

Carson said that, at a meeting attended by Somali President Sheikh Sharif Ahmed, the United Nations special representative for Somalia Augustine Mahiga said he was against allowing AMISOM to attack al Shabaab.

Carson did not say whether the U.S. supported AMISOM going on the offensive.

Diplomats at the summit told Reuters the heightened rhetoric from both African and Western countries in the wake of the bombings might be a sign the AU was ready to give AMISOM permission to hunt down al Shabaab.

Some countries are against the policy change, citing evidence that AMISOM has killed civilians. They argue such incidents could be a recruitment boon to the rebels, who control parts of the capital Mogadishu and much of southern Somalia.

"Somalia is a source of terrorism which has been visited upon countries like Tanzania, Kenya and most recently Uganda," Carson said. "It is also a place where we see foreign fighters and an increasing number of extremists operating."

At least 21,000 Somalis have been killed in fighting since the start of 2007, 1.5 million have been uprooted from their homes and nearly half a million are sheltering in other countries in the region.

The summit will on Tuesday pass its resolution on Somalia.

Source: Reuters