The world cannot watch the spread of "evil" from the Islamic State militant group, US Secretary of State John Kerry has said in a news conference in Iraq.
Mr Kerry, on a Middle East tour, said IS was the "single greatest threat" the people of Iraq now faced.
He said there would be a global plan to defeat IS, but the new Iraqi government must be the "engine" of that fight.
IS has seized large areas of northern Iraq, overrunning the army and gaining backing from some disaffected Sunnis.
Meanwhile, at least 13 people died in a series of car bombs in east Baghdad.
The explosions happened minutes apart in the Shia neighbourhood of New Baghdad. At least 20 others were injured.
Barbara Plett-Usher, BBC News, Baghdad
An inclusive Iraqi government was the condition the Americans set for increased US and Nato military support. But a senior state department official said Mr Kerry was mostly talking to leaders about Baghdad's role in the global coalition against Islamic State.
The US wants to see speedy moves by the Shia majority to meet the grievances of the disgruntled Sunnis and Kurds, so all can form a united front against IS.
That is an enormous challenge, given that many disputed issues have deep structural roots. But the US official said a significant step would be the creation of regional armed forces or national guard units, a decision taken in the wake of criticism that the army had been used as a sectarian tool by the former Shia prime minister.
Mr Kerry made an unannounced visit to the Iraqi capital Baghdad at the start of a tour of Middle Eastern capitals aimed at boosting military, political and financial support for the fight against IS.
He held talks with Iraq's new Prime Minister Haidar al-Abadi, who appealed for international support to defeat the "cancer" of IS.
"Of course, our role is to defend our country, but the international community is responsible to protect Iraq," Mr Abadi said after meeting Mr Kerry.
The two men also discussed Mr Abadi's plans for his new government.
The prime minister, a Shia, has promised to devolve more power to Sunnis and tackle the central government's relations with the Kurdish minority.
He has also said he will overhaul the military and make it more inclusive.
The US secretary of state said he was "very encouraged" by Mr Abadi's commitment to reform.
"A new and inclusive Iraqi government has to be the engine of global fight against [Islamic State]," Mr Kerry said.
The previous Shia-led government of Nouri Maliki was accused of exacerbating sectarian tension by excluding Sunnis from power and failing to deal with Kurdish demands.
Where regional powers stand
- Iran: Calls for co-operation against IS, turns a blind eye to US military action
- Syria: Assad government opposed to IS, but US prefers to support moderate rebel groups fighting them
- Turkey: Opposes IS, but opposition muted by concern for 49 Turkish diplomats and their families kidnapped by IS in Mosul in June
- Saudi Arabia: Key supporter of Syrian rebels, including Islamists, but denies direct support for IS
US President Barack Obama is due to give details later of the US strategy to tackle IS.
He said on Tuesday that he had authority to widen military action against IS without the approval of Congress.
The US has already conducted dozens of air strikes on IS targets in the past month, in an effort to protect ethnic and religious minorities threatened by IS.
IS militants described the beheadings of two US journalists as a retaliation against the strikes.
Powerful militant Shia cleric Moqtada al-Sadr appeared to criticise Mr Abadi for meeting Mr Kerry.
"We wish for Iraq to co-operate with the neighbouring countries and its allies, but not with the occupiers," he was quoted as saying by Reuters news agency, referring to the recent US military presence in the country.
Meanwhile, Saudi ambassador in London Prince Mohammed bin Nawaf rejected suggestions that his government was supporting or funding IS.
He instead blamed a lack of international involvement for allowing the group to flourish.
The White House said Mr Obama spoke to Saudi Arabia's King Abdullah about IS on Wednesday.
Analysts say the kingdom, which has backed Sunni groups around the Middle East, is crucial to any effort to build a coalition against IS.