US issues travel alert for Kenya, citing al-Shabaab

* Kenya, Somali rebels clash on the border

* Potential for unrest as referendum nears

By JoAnne Allen

WASHINGTON, July 22 (Reuters) - The U.S. State Department issued a travel warning on Thursday for Americans in Kenya, citing increasing threats in the wake of deadly suicide bomb attacks earlier this month in neighboring Uganda.

The U.S. Embassy in the Kenyan capital urged U.S. citizens to be cautious in public areas and warned them to avoid public rallies and demonstrations.

"Since the July 11 bombings in Uganda, for which the Somalia-based, U.S.-designated Foreign Terrorist Organization al-Shabaab terrorist group has claimed responsibility, there have been increased threats against public areas across East Africa," the State Department said.

The U.S. warning follows a clash this week between Kenyan security forces and gunmen from the Somali rebel group along the border with Somali.

Both Kenya and the al Shabaab militia were reported to be sending reinforcements to the area, although similar border skirmishes in the past have not escalated into wider fighting.

Al Shabaab, a hardline Islamist rebel group with links to al Qaeda, controls much of southern Somalia, bordering northeastern Kenya, and is fighting to topple the Western-backed government in the Horn of Africa nation.

The Ugandan bombings were the first al Shabaab strike outside Somalia, although there have been fears for some time in Kenya that the violence could spill over the long and porous border between the two countries.

The State Department also warned of the potential for civil unrest ahead of Kenya's upcoming constitutional referendum.

"While the U.S. Embassy hopes for a peaceful August 4 constitutional referendum with high voter turnout, it recognizes that past elections in Kenya have been marred by violence," the State Department said.

Six people died in a grenade attack at a political rally last month.

The proposed charter is the centerpiece of a deal signed to end the violence that followed Kenya's 2007 presidential election, which killed at least 1,300 people, scared off investors and tourists and sent growth rates plummeting. (Reporting by JoAnne Allen; editing by Todd Eastham)