(Reuters) - U.S. lawmakers on Monday pushed for more sanctions against Iran after talks between Tehran and global powers failed to stop Iran from developing its nuclear program.

Although the talks between Iran, the United States and five other world powers were described as "constructive" by the European Union´s foreign policy chief, Catherine Ashton, some U.S. lawmakers said they were unimpressed.

"The United States should not mistake positive diplomatic dialogue for compliance with United Nations Security Council resolutions," said a spokesman for Republican Senator Mark Kirk.

Kirk and several other U.S. legislators have been pressuring the White House to get tougher on Iran and are pushing for a range of additional penalties that would further isolate Iran and prevent it from trading with the rest of the world.

President Barack Obama warned on Sunday there would be more sanctions imposed on Iran if there was no breakthrough in talks in coming months.

Iran´s foreign minister said Tehran was ready to resolve nuclear issues if the West starts lifting sanctions.

FIVE WEEKS

U.S. sanctions that Obama signed into law in December have already forced some of Iran´s biggest trading partners, such as Japan, to reduce their Iranian oil imports.

Other countries are scrambling to cut purchases of Iranian crude before a mid-year deadline. If they fail to do so, those countries could see their banks blocked from U.S. markets.

"We have five weeks to convince the Iranians that the sanctions we passed in December were only a first step," said Democratic Senator Robert Menendez, who along with Kirk helped design the sanctions that were signed into law in December.

Iran and the group of world powers, which comprises the five permanent members of the U.N. Security Council - the United States, Russia, China, France and Britain - plus Germany, have agreed to reconvene talks in Baghdad May 23.

Ashton, who leads the negotiations for the six global powers, has said she expects subsequent meetings would lead to concrete steps "towards a comprehensive negotiated solution which restores international confidence in the exclusively peaceful nature of the Iranian nuclear program."

Many Republican and Democratic Senators support legislation that would force United States to target Iran´s main oil and shipping companies and require publicly traded companies to disclose their Iran-related activities.

But the legislation stalled when Senate Majority Leader Harry Reid refused to allow lawmakers to consider adding further penalties to the bill.

A spokesman for Reid said he had not decided whether to bring the bill to the Senate floor during the current session. Reid has chastised Republicans for blocking the bill although there has been bipartisan support for further measures.

Menendez said it was crucial that Congress pass the legislation quickly to send a message to the Iranian government that the United States "won´t allow them to use the Baghdad talks to stall for more time to advance their covert nuclear program."

(Reporting By Roberta Rampton, Rachelle Younglai; editing by Christopher Wilson) (© Thomson Reuters 2012. 04/16/12)