LONDON, OCT 13 (AP/UNB) - The alleged plot to kill Saudi Arabia's ambassador to the U.S. marked an escalation of Iran's sponsorship of terrorism overseas, Britain's foreign secretary said Thursday.

William Hague told lawmakers that Britain was working with the U.S., Saudi Arabia and others to agree on a tough international response to the purported plan to assassinate the envoy in a bombing while he dined at his favorite Washington restaurant.

FBI Director Robert Mueller has said many lives could have been lost in the plot to kill Saudi envoy Adel Al-Jubeir.

The plot appeared to be a "major escalation of Iran's sponsorship of terrorism outside its borders," Hague told the House of Commons in a statement on the Middle East.

A U.S. criminal complaint accuses two Iranian suspects of hiring a would-be assassin in Mexico who was a paid informant for the U.S. Drug Enforcement Administration and told U.S. authorities about the details of their plot.

Manssor Arbabsiar, a 56-year-old U.S. citizen who also holds an Iranian passport, was charged along with Gholam Shakuri, who authorities said was a Quds Force member and is still at large in Iran.

Britain is working with allies to agree on new sanctions - likely to include travel bans and asset freezes - against those linked to the plot, Prime Minister David Cameron's office has said.

However, diplomats have indicated Britain would not support any calls for military action.

Hague also told lawmakers he had summoned Syria's ambassador to a meeting to challenge him over allegations of intimidation of anti-regime protesters in Britain.

He said he had made clear to Syrian ambassador Sami Khiyami that harassment of those demonstrating against President Bashar Assad's regime was not acceptable.

London police are investigating allegations that Syrian Embassy staff have photographed people protesting outside the country's embassy, and shown the images to the demonstrators' families in Syria.

Hague told legislators that a meeting of Britain's National Security Council on Thursday had discussed allegations of human rights abuses against Libyans who remain loyal to Moammar Gadhafi.

Cameron's spokesman Steve Field said that John Jenkins, Britain's envoy in Libya, would remind the National Transitional Council in Libya "that they need to deliver on their commitment to hold all people who commit human rights abuses to account."