By Sam Youngman

RENO, Nevada (Reuters) - Republican presidential candidate Mitt Romney accused President Barack Obama on Tuesday of weakness in the face of U.S. adversaries and promised he would be firm against Iran and China as he prepares to head overseas to boost his foreign policy credentials.

The former Massachusetts governor visits Britain, Israel and Poland this week to try to disprove Democratic accusations that he is inexperienced abroad.

As U.S. presidential candidates often do, Romney vowed to maintain America's leading role in the world. But his comments were among the most direct yet in his criticism of Obama's handling of Washington's allies and foes.

"This is very simple: If you don't want America to be the strongest nation on Earth, I am not your president," Romney told a convention of the Veterans of Foreign Wars. "You have that president today."

Speaking in an animated way, the Republican blamed his November 6 election rival for a cooling of relations with Israel and for "shabby treatment" of the key U.S. ally in the Mideast.

Although U.S. voters remain focused on the economy, international issues like the war in Afghanistan, the conflict in Syria and the growing influence of China make for a critical backdrop in the campaign for the November 6 election.

Romney promised a hard line against Iran to prevent it from producing a nuclear weapon and said, "There must be a full suspension of any enrichment, whatsoever, period."

"It is a mistake - and sometimes a tragic one - to think that firmness in American foreign policy can bring only tension and conflict," Romney said.

"PHOTO-OPS" TRIP

Obama's campaign dismissed Romney's coming foreign trip, including a visit to the Olympic Games in London, as lacking in substance, underscoring the Democrats' effort to depict him as a foreign policy lightweight.

Obama campaign spokeswoman Jen Psaki said that according to Romney's schedule he is going overseas "to do some fundraising and he has some photo-ops."

"These several days are not going to help him jump over that bar and convince the American people he wants to have a serious conversation about foreign policy," she said.

Romney pledged to take a tough line against China and Russia, and blamed the White House for national security leaks to the media about the raid that killed al Qaeda leader Osama bin Laden and cyber-warfare against Iran.

"What kind of White House would reveal classified material for political gain?" Romney said. "I'll tell you right now: Mine won't."

Attorney General Eric Holder appointed two chief federal prosecutors last month to spearhead an investigation into suspected leaks of classified information amid allegations the White House made the disclosures to boost Obama's re-election chances.

"This conduct is contemptible," Romney said. "It betrays our national interest. It compromises our men and women in the field. And it demands a full and prompt investigation, with explanation and consequence," Romney said.

The two candidates are running close in opinion polls but surveys often give Obama relatively high marks for his handling of foreign policy, helped by the killing last year of bin Laden. Romney is searching for a way to hit Obama on national security.

"Given the inability of Romney to find a contrast on foreign policy that works toward his election, he is attacking on alleged leaks in the hope that something will stick to Obama's record as commander in chief," said political analyst Dante Scala, a professor at the University of New Hampshire.

In his speech, Romney also criticized the "sequestration" trigger Obama and congressional Republicans agreed to last summer.

The deal puts in place an automatic $1 trillion budget cut - half of that in defense spending - that Romney says is an "arbitrary, across-the-board budget reduction that would saddle the military with a trillion dollars in cuts, severely shrink our force structure and impair our ability to meet and deter threats."

Obama made a speech to the VFW meeting on Monday and was generally well received as he made a similar call to Romney's on sequestration and urged Republican legislators to stop "playing politics" with the U.S. military budget.

(Additional reporting by Alister Bull and Margaret Chadbourn; Editing By Alistair Bell and Philip Barbara)