LONDON - The United States is seeking European support for tougher sanctions on Iran in response to an alleged plot to kill the Saudi ambassador to Washington, a senior US official said on Monday.

David Cohen, Treasury undersecretary for terrorism and financial intelligence, said he met British officials in London on Monday to discuss potential new sanctions focusing on Iran's access to the global financial system.

"Iran needs to be held accountable for this plot," Cohen said, referring to an alleged plan to assassinate the Saudi ambassador in a bomb attack, in which the Iranian government denies any involvement.

Any further sanctions would also be part of efforts to deter Iran from pursuing nuclear capabilities, Cohen added, and could target the country's central bank.

"We are going to continue to look at those financial institutions that are involved with proliferation activity for Iran and continue to try to isolate them from the international financial sector," Cohen said.

Manssor Arbabsiar, 56, an Iranian-American man who US officials say has links to Iran's security forces, pleaded not guilty in a federal court in New York on Monday of plotting to kill the Saudi ambassador.  

Another man, Gholam Shakuri, was also charged in the plot but is believed to be in Iran.

The European Union, which imposed sanctions against five men associated with the alleged plot on Friday, warned Iran on Sunday it could face harsher measures if it failed to address concerns about its nuclear programme.

Iran has been subjected to four rounds of sanctions by the United Nations since 2006. Further UN sanctions would require the backing of permanent Security Council members Russia and China, whose support is far from certain.

Western powers accuse Iran of trying to develop a nuclear weapon, but Tehran insists its programme is peaceful.

Western diplomats in Vienna say they expect the UN nuclear watchdog to publish intelligence next month pointing to military dimensions to Iran's nuclear activities but stopping short of saying explicitly that Tehran is trying to build atom bombs.

Cohen will now travel to Berlin, Paris and Rome to discuss potential sanctions with other European governments, both on Iran and on Syria, facing international condemnation over its bloody repression of protests.

Speaking at the US Embassy, Cohen said the United States and the EU were aligned on imposing "additional sanctions" on Syrian human rights violators, and on members of the Syrian government.  

The EU blocked crude imports from Syria in September and imposed sanctions on companies in protest at the government's crackdown.