WASHINGTON (Reuters) - U.S. President Barack Obama's drive for tougher sanctions on Iran gained momentum on Monday in talks with Chinese President Hu Jintao that also focused on their countries' fractious economic relationship.

Obama stressed to Hu the need to act urgently against Iran's nuclear program, and Hu agreed that Beijing would help craft a U.N. resolution, a U.S. official said afterward.

Their 90-minute encounter came at the start of an unprecedented two-day summit of nearly 50 countries that Obama has called to highlight the global threat of nuclear terrorism and to agree an action plan to prevent weapons-grade atomic material from falling into the hands of terrorists.

Ukraine provided the first example by agreeing to give up its highly enriched uranium.

Iran's nuclear program, which the West fears is a cover to build an atomic bomb, is not on the agenda of the summit, but the presence of so many world leaders in one place gave Obama an opportunity to again make his case for fresh sanctions to be imposed on Tehran over its refusal to halt uranium enrichment.

China has close economic ties with Iran and has so far been reluctant to agree to tougher sanctions. U.S. and Chinese officials who briefed reporters after the Hu-Obama talks described a positive, constructive atmosphere on Iran.

Hu told Obama that China and the United States shared the same overall goal on reining in Iran's nuclear program, Chinese Foreign Ministry spokesman Ma Zhaoxu said.

Ma's comments confirmed China's recent decision to join discussions with world powers on Iran but they did not indicate a new willingness to embrace harsher sanctions, such as ones that would target the Islamic Republic's energy sector.

Ma also repeated China's standard call for "dialogue and negotiations" with Iran.

Hu's agreement to attend the summit was perceived as a positive sign in Washington after U.S.-Chinese relations were strained by Obama's meeting with the Dalai Lama, China's Internet censorship, and U.S. pressure over China's currency.

U.S. Treasury Secretary Timothy Geithner and White House economic adviser Larry Summers attended the meeting with Hu, in which Obama also raised U.S. concerns about China's currency, the yuan, and urged the country to move to a more market-oriented exchange rate, a U.S. official said.

Hu told Obama economic and trade frictions between the two countries should be resolved through consultations "on an equal footing," Ma said.

Washington has been pressing Beijing to lift the value of the yuan, and many U.S. lawmakers say that by deliberately holding down its currency China is giving its firms an unfair export subsidy that costs jobs in many countries.

The United States recently delayed a decision on whether to declare China a currency manipulator, a move that would have potentially jeopardized Hu's attendance at the summit. U.S. officials have denied any link between Hu's visit and the decision.

IRAN DISMISSES SUMMIT

Iran dismissed the U.S. summit and said it would not be swayed by any decisions made there. "World summits being organized these days are intended to humiliate human beings," Iranian President Mahmoud Ahmadinejad said in Tehran.

Obama spent Monday holding a series of meetings with foreign leaders before the serious business of the summit gets under way on Tuesday, when delegations representing 47 nations will gather to discuss how to combat nuclear terrorism.

The summit in Washington's downtown convention center, which was surrounded by a heavy security cordon of troops and police and high fences, is the culmination of a hectic period of nuclear diplomacy for Obama.

Last week he signed a new treaty to cut U.S. and Russian nuclear arsenals and unilaterally announced the United States would limit its use of nuclear weapons, a plan that came under heavy fire from his conservative critics.

The summit -- the biggest U.S.-hosted assembly of world leaders in six decades -- will be a test of Obama's ability to rally global action on his nuclear agenda.

It had its first tangible outcome when Ukraine announced it would give up its stockpile of highly enriched uranium by 2012, most of it this year. Kiev has enough nuclear material for several weapons. It will convert its civil nuclear program to operate on low-enriched uranium.

Canadian Prime Minister Stephen Harper pledged to return a "significant quantity" of Canada's spent nuclear fuel to the United States, the original supplier, by 2018.

A draft final communique shows leaders will pledge to work toward safeguarding all "vulnerable nuclear material" within four years and take steps to crack down on nuclear smuggling.

The list of leaders in attendance ranged from heads of state of traditional nuclear powers like Russia and France to nuclear-armed foes like India and neighboring Pakistan.

(Writing by Steve Holland and Ross Colvin; additional reporting by Paul Eckert, Dan Williams, Jeff Mason, Patricia Zengerle and Alister Bull in Washington and Ramin Mostafavi in Tehran, Editing by David Storey and Eric Walsh)