WASHINGTON – Diplomats from six world powers arrived in Moscow on Sunday facing a double challenge: to coax concessions from Iran over its disputed nuclear program, and to keep the negotiations from collapsing if Tehran refuses.

After two previous rounds, the United States and other negotiators preparing for talks today and Tuesday still don’t understand Iran’s intentions. They say Tehran continues to send confusing signals about whether it is willing to compromise. The alternative is more talk of war and an oil shock to the world economy.

The longer the diplomacy drags on, the more Iran will suffer from sanctions — but, at the same time, the more uranium it can enrich.

For now, the diplomatic action is moving at glacial speed. But few expect negotiations to come to a complete halt.

Iran’s chief nuclear negotiator, Saeed Jalili, promised European Union foreign policy chief Catherine Ashton this month that Iran will address a proposal for an interim deal that would require Iran to immediately suspend enrichment of uranium to a purity that could be converted relatively easily for use in nuclear bombs.

Jalili ignored the proposal when he met last month in Baghdad with the six negotiating powers — Britain, France, Germany, Russia, China and the United States — so his comment was seen as a sign of a potential breakthrough at the Moscow talks.

Yet Jalili also told the legislature in Tehran that Iran won’t compromise on its right to enrich uranium. He said he expected the six powers to address a list of Iranian grievances on topics unrelated to the nuclear program, a nonstarter for Washington and its allies.

In another troubling sign, Iran this month hit an impasse with the United Nations’ nuclear watchdog, the International Atomic Energy Agency, over a proposed deal that would have allowed the agency to broaden its often-frustrated inspections of Iranian nuclear facilities.

The slow progress has stirred talk in the group of pausing the talks to allow a European Union oil embargo and stiff new U.S. sanctions due to begin on July 1 to further strangle Iran’s already-battered economy.

But even a temporary suspension of diplomacy carries risks. It could upset nervous oil markets and raise fresh anxieties about a military strike by Israel, which has warned that it may bomb Iran’s nuclear facilities if diplomatic efforts fail.

Key lawmakers in Washington also signaled that they are losing patience with the lack of progress and threatened to impose even more crippling sanctions on Tehran.

On Friday, a bipartisan group of 44 senators signed a letter to President Obama, urging him to step up demands and to “re-evaluate the utility of further talks” if no “substantive” progress is reached in Moscow.

“Moscow has to produce some results, or we should come down like a ton of bricks on Iran,” said U.S. Rep. Howard Berman, D-Calif., the ranking minority member of the House Foreign Affairs Committee.