The U.S. Congress took steps on Tuesday to tighten sanctions against Iran in what many lawmakers call an effort to provide President Barack Obama the authority he needs to increase pressure on Tehran over its nuclear program. One week after U.S. Secretary of State Hillary Clinton told Congress that the Obama administration will work with lawmakers to impose "crippling" sanctions against Iran, Senate lawmakers introduced legislation designed to help accomplish just that.

In an effort similar to action in the House of Representatives, a bipartisan group of senators introduced legislation targeting Iran's gasoline imports, including companies supporting Iran's energy sector or insuring fuel shipments to Iran.

Sen. Joseph Lieberman (file photo)

Senator Joseph Lieberman, who calls himself an Independent but caucuses with Democrats, says the legislation is part of a process in which the United States and its allies "turn the page" toward finding more common ground on Iran and setting specific benchmarks with global enforcement.

"What we need is a multi-pronged, explicit strategy that employs all of the elements of our national power and our allies, and that ties together multiple lines of operation, including direct diplomacy with the Iranians, into a coherent plan of action for the months ahead that has goals and schedules and teeth," he said.

Lieberman says Iran must understand that it is on an explicit timeline in which the international community expects to see results. The Obama administration, he added, must make clear that it does not view engagement with Iran as a process without an end, but as a means to identify ends that would benefit both countries.

Like the House version, the Senate measure would amend the 1996 Iran Sanctions Act to allow the president to sanction foreign companies involved in enabling the shipment of gasoline and other refined petroleum products to Iran, or helping to maintain its existing refining capacity.

Proposals to sanction Iran's gasoline imports, which account for 40 percent of its needs, have been advocated by various lawmakers in recent years.

Meanwhile, the House Financial Services Committee took a step to make it easier for U.S. state governments and individual investors to divest from Iran, removing the threat of lawsuits for those doing so.

"There are people who will think that it is important that we tighten sanctions against Iran; there are some who may feel the time has come to relax them. This bill doesn't take sides. What it says is that it is inappropriate to have a federal rule that says state governments with their own money can't make these decisions or that individual investors can't press their companies to do it," said House Financial Services Committee Chairman, Democrat Barney Frank.

The House measure is similar to legislation Congress approved regarding Darfur in Sudan, but would not direct the federal government to publish a list of companies with investments of more than $20 million in the Iranian energy sector -- a provision that drew opposition from some lawmakers.

Ten U.S. states have enacted Iran divestment legislation, and an amendment to the House bill would retroactively expand protection to those actions.

Republican Jeb Hensarling says changes agreed to by Democrats and Republicans have improved the measure, adding that it should attract strong bipartisan support when it comes up for a vote in the House.

Iranian officials this week warned against attempts to sanction gasoline imports.

In separate action, the House of Representatives debated a resolution urging Iran to step up cooperation with the U.S. and intermediaries on the case of Robert Levinson, the former FBI agent who disappeared in 2007 while visiting Iran.

It expresses appreciation for Iran's "promise of continued assistance" but urges President Obama and U.S. allies to raise the case at every opportunity, "notwithstanding other serious disagreements with Iran over its nuclear program and other issues".

A similar concurrent resolution is pending in the Senate. Such measures express sentiments of both chambers of Congress, but do not have the force of law.