"Engagement must be serious and credible, but it cannot be open-ended," the group, which included chairman of key House committees, wrote in the letter amid growing signs of Washington outreach to the Islamic republic.

The lawmakers said the central goal of any talks must be to get Iran to verifiably suspend uranium enrichment -- a possible prelude to a nuclear weapon -- "within at most a few months of the initiation of discussions."

They said US outreach "cannot be deferred" until after Iran's June presidential elections and said Washington and its allies must be prepared to impose a range of painful economic sanctions in Iran pursues enrichment.

"We urge that the talks begin as soon as possible, so that we will have the earliest possible indication of whether they will succeed in halting Iran's nuclear program," they wrote in the message.

Obama must be prepared to consider moves like slapping sanctions on Iran's central bank, punishing international banks that do business with Iranian banks, targeting energy firms that invest in Iran's oil and gas sector, and closing US ports to shipping companies that call on Iranian ports.

US allies "should be prepared to prohibit Iranian banks from operating on their territory and to cease allowing Iran to conduct international financial transactions in Euros, pounds, and yen," the lawmakers wrote.

US allies must also stop providing export credits to firms seeking to do business in Iran, and bar exports of refined petroleum and catalytic converters to the Islamic republic, they said.

The seven lawmakers also urged Obama to "make Iran a matter of the highest priority in our relations with Russia and China.

The group included Democratic House Majority Leader Steny Hoyer, as well as the Democratic chairmen of the House Foreign Affairs, Armed Services, and Intelligence Committees.

"If we truly mean that Iran cannot be allowed to possess nuclear weapons, urgent action is required today. We must use the time available to us to begin engagement and to convince the Iranians of our seriousness of purpose. Otherwise, we will face far more difficult decisions in the future," they said.

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