When President George W. Bush last week addressed a joint session of Congress, he spelt out in no uncertain terms what he expected from the rest of the world.

"Every nation, in every region, now has a decision to make," he said. "Either you are with us or you are with the terrorists. From this day forward, any nation that continues to harbour or support terrorism will be regarded by the United States as a hostile regime."

Many countries have since rushed to support an administration intent on launching an unprecedented mission against terrorism. But support is not so clear-cut. Some are concerned the US will use this campaign to bypass the United Nations and impose a new world order. Others fear domestic instability if they give unconditional support.

At the forefront in solidarity with the US is the European Union. At an extraordinary summit of its leaders last Friday, EU leaders said any US response to the bombings was legitimate. They would co-operate on every level, they said. They also pushed through a package of anti-terrorism measures, including a European arrest warrant.

Some European governments, including candidate states, have also offered the US use of their air space and intelligence and several have suggested they would offer troops if asked. Outside Europe, Japan, Canada, Australia and New Zealand have made similar offers. China, too, has promised to share intelligence.

Despite domestic opposition, Pakistan agreed to help the US track down Osama bin Laden, the prime suspect behind the bombings. But there are countries that want nothing to do with this global coalition, including North Korea and Iraq, often referred to as "rogue states" by Washington.

However, between states fully on board and those outside the coalition is a grey world of countries, which have condemned terrorism but remain uneasy about supporting the US.

Mahathir Mohamad, Malaysia's prime minister, said Mr Bush's war on terrorism overlooked the "anger among oppressed peoples" such as Palestinians, Chechens and Iraqis. Iran has indicated it wants to remain neutral - and will therefore not offer air space to US war planes. It also fears that US strikes on Afghanistan will provoke a mass exodus of refugees in addition to the 1.5m already in Iran.

The Central Asian republics of Uzebekistan, Kyrgyzstan and Tajikistan are also in an awkward position. Not only do their borders flank Afghanistan, they share an ethnic and religious identity with tribes in northern Afghanistan.

Any bombing of that country could unleash a backlash among Islamic populations there.

Russia, too, is in a grey zone but an exclusive one. It is in a powerful position to makes demands on the US in return for substantial support. Some diplomats say Mr Putin could have a long shopping list for Mr Bush, ranging from concerns about Washington's missile defence system, blocking Nato from expanding to the Baltic states and compensating Russia if it stopped exporting arms to Iran.

But it is the Arab world that remains the most ambiguous towards Mr Bush's war on terrorists - partly because the US is so closely identified with Israel.

Saudi Arabia downgraded its relations with the Taliban some time ago and the United Arab Emirates on Sunday severed its ties.

Elsewhere, Arab leaders support anti-terrorist measures but several states would prefer any coalition to be under the aegis of the UN. Moreover, Muslim groups fear their governments will use anti-terrorist measures as an excuse to crack down further on the Islamic opposition without addressing the conditions that often fuels radicalism.