By Mark Gregory
BBC News, Washington
The US is threatening to get much tougher with international energy companies that do business with Iran.
The US could penalise major oil and gas firms
Oil firms may face fines and other penalties if they sign deals to develop Iranian reserves of oil and gas, a State Department source told the BBC.
The statement marks an escalation of US financial pressure aimed at persuading Tehran to abandon alleged plans to develop nuclear weapons.
Iran insists its nuclear programme has no military application.
A law already on the statute book gives the US government wide ranging authority to penalise companies doing business with Iran.
Under the Iran Sanctions Act Washington can fine not just American firms, but any company anywhere that does more than $20m business with Iran.
But so far the US government has generally chosen not to use these powers.
However, this stance may be about to change.
Nicholas Burns, one of the most senior officials at the State Department, has a blunt message for energy companies considering Iranian deals.
"We have been going round to the major oil and gas firms to let them know that this law exists and that we implement it if they cross the line," he said.
The Bush administration has come under pressure from Congress, now Democrat controlled, to take a stronger line with companies trading with Iran.
Mr Burns, when asked if he was talking tough simply to keep the US legislature happy, said: "I wouldn't assume it. We have a requirement to enforce the law".
A number of European energy giants - including Shell, Repsol of Spain and France's Total - are at the moment considering multi-billion dollar contracts to develop Iranian gas fields.
Chinese and Malaysian energy groups are also showing interest in Iran.
The US government is none too subtly warning them all to back off now.