Oil prices simmered close to their highest level in a year on Monday as the United States stepped up diplomatic efforts to win support for toppling Iraqi leader Saddam Hussein.

The United States and Britain expressed concern over the weekend that Saddam was developing nuclear weapons and said there was sufficient evidence to act.

International benchmark Brent crude oil rose six cents to $28.38 a barrel, near its highest level in a year. U.S. crude futures slipped two cents to $29.59.

"Crude has remained firm following increased rhetoric from the U.S. and UK against Iraq," said Lawrence Eagles of brokers GNI Research.

Brent has jumped 43 percent so far this year on growing fears of a war in the Gulf region, which pumps a quarter of the world's daily diet of 76 million barrels.

Western oil importing countries say crude prices around $30 could jeopardise the fragile global economic recovery by increasing energy costs.

They are pressing the Organization of the Petroleum Exporting Countries to release more supply onto world markets at a policy meeting set for Sept 19.

The "war premium" for uncertainty over Iraq has added between $4 and $6 on a barrel of crude, analysts believe, which will give OPEC price hawks ammunition to argue against any relaxation of supply in the fourth quarter.

U.S. crude topped $30 last week as a new U.S.-British attack on Iraqi air defense positions raised speculation that an all-out attack against Saddam was imminent.

A string of top advisers to President Bush cited new evidence on Sunday that Saddam was trying to make a nuclear bomb.

Bush plans to address the United Nations on Thursday and some expect him to deliver an ultimatum to Saddam to accept weapons inspectors or face war.

OPEC oil output is running at its lowest level in a decade after four successive cuts since the start of 2001, when the world economy began to slide.

Price hawks Kuwait, Iran, Indonesia and Venezuela want current curbs extended unless prices rise further, but analysts believe kingpin Saudi Arabia wants to release more oil to prevent a spike this winter.