Republican presidential candidate to visit UK, Israel and Poland, and Obama campaign says he must spell out his plans

Mitt Romney will arrive in London on Wednesday pursued by calls from the Obama administration to use his first overseas tour as the Republican presidential candidate to define a foreign policy that amounts to more than criticising the president.

Mocked for his description of Russia as the US's leading geopolitical foe and criticised for "posturing" over a military attack on Iran, Romney hopes to use his visits to Britain, Israel and Poland over the next week to counter the perception that he has a tenuous grip on foreign affairs.

Convention has it that contenders for the White House do not attack the president while on foreign soil, but the Obama campaign invited just that when it challenged Romney to use two scheduled speeches to spell out what he would do differently.

"I don't know how you give a major foreign policy speech and not give the policy details," said Robert Gibbs, the former White House spokesman and now an Obama campaign adviser. "The bar really is whether or not Mitt Romney is finally ready to shed a little light on what appears to be the secrecy of his foreign policy plans."

The Romney campaign said the three countries on the tour itinerary were chosen as "pillars of liberty" and for their strong ties to the US. It said the intent was to demonstrate a resolute stand with places that share America's values – a hint at the Republican contender's claim that Obama has let down Washington's friends abroad while offering grovelling apologies to its enemies.

In London Romney's meetings with the prime minister, David Cameron, his deputy, Nick Clegg, and the Labour leader, Ed Miliband, will permit him for the first time to project himself as a politician capable of reaching beyond his limited political experience as a state governor.

Romney will also meet the chancellor, George Osborne – a chance to ally himself with an economic strategy of deep cuts to public spending.

The Republican contender plans to attend the opening ceremony of the Olympics, providing an opportunity to remind US voters of his claim to have saved the 2002 Salt Lake City winter Games from meltdown.

His trip to Israel is the most likely to have political impact back home, not least as Romney attempts to rally conservative Christian evangelical voters who are strong supporters of Israel but sceptical about his Mormon faith.

Romney's team said he was going to "learn and listen". But the Obama campaign is piling pressure on Romney to clarify his criticisms of the president's approach to Iran's nuclear programme. It has homed in on Romney's claim that Obama is not strong enough in confronting Tehran and too weak in failing to promise unconditional support for Israel if it attacks Iran.

Colin Kahl, a former defence department official in the Obama administration, challenged Romney for failing to explain what he would do that is different to the White House's approach of putting an emphasis on sanctions and diplomacy while saying the use of force remains a final option.

"This isn't the time for anyone to be playing politics with our policy in the region," Kahl said. "If Romney thinks it's time to use military action against Iran and abandon diplomacy this early, I think he owes it to the American people to actually say so."

That may be a hard subject to avoid in Jerusalem. The Israeli prime minister, Binyamin Netanyahu, will be looking for support on his Iran policy, which is rooted in deep scepticism that sanctions and diplomacy will work. Israel says there is a rapidly shrinking window in which to carry out an effective military attack against Tehran's nuclear facilities. But there is little evidence of an appetite among most US voters for a conflict with Iran.

Romney has also derided Obama's testy relationship with Netanyahu as endangering Israel's security. He has criticised the president for public differences with the Israeli leader over the expansion of Jewish settlements in the occupied West Bank.

Support for Netanyahu on the settlements and the lack of progress on an agreement with the Palestinians will play well with Christian evangelicals in the US but is not likely to do much to enhance Romney's foreign policy credentials more broadly.

There is little doubt that the Israeli prime minister would prefer to see Romney in the White House. They have known each other since the 1970s when they both worked at the Boston Consulting Group.

But it remains to be seen if Netanyahu will risk further alienating Obama with tacit shows of support for Romney without being confident of a Republican victory.

Romney has also attempted to portray Obama as weak over the president's plan to withdraw US troops from Afghanistan in 2014 and for his response to the uprising in Syria.

The Republican candidate said Obama was right to call for the Syrian leader, Bashar al-Assad, to quit but the president had not done enough to force it, relying on international diplomatic pressure and allowing Russia to block stronger action.

"I think from the very beginning we misread the setting in Syria," Romney told CNBC. "America should have come out very aggressively from the very beginning and said Assad must go … the world looks for American leadership and American strength."

Michele Flournoy, a former undersecretary of defence in the Obama administration, challenged Romney to say how he would address these complex foreign policy issues.

"We face a number of challenges going forward and they can't be faced with political talking points and platitudes and criticising your opponent," she said. "They really require serious people with serious ideas and people who understand the complexity of the world we live in. Romney hasn't really told us yet how we would confront these challenges."

The Republican camp quickly hit back.

"In no region of the world is our country's influence any stronger than it was four years ago," said Romney's spokesman, Ryan Williams. "President Obama has failed to restore our economy, is weakening our military with devastating defence cuts, and has diminished our moral authority."

Romney ends his tour in Poland at the invitation of its former president, Lech Walesa, a founder of the Solidarity movement that brought down the country's communist government.