Achieving political unity in Iraq is "the single most important factor" in addressing the "existential threat" posed to the country by Isis extremists, William Hague told its leaders after holding talks with prime minister Nouri al-Maliki and other key figures in Baghdad.
The Foreign Secretary appealed for sectarian differences to be put aside and an inclusive government to be formed in the interests of the "stability and freedom" of Iraq and other countries on the first day of his visit.
Mr Maliki - who is criticised for favouring the interests of the Shia population - has already insisted that introducing an emergency administration would amount to a "coup against the constitution" after he won elections in April.
Mr Hague also said the UK would press for action at the United Nations to stem the flow of arms from insurgents and cut their access to finance, and pledged to use "the full force of the law" to stem the flow of British citizens to he region to join the fight.
After what he called a "very useful discussion with the national security adviser" about the activities of Iraqi security forces - who in some areas put up little or no resistance to the Isis surge - he said: " The message from the United Kingdom is very clear: we urge all Iraq's leaders and communities to unite.
"The Iraqi state faces an existential threat, and the growth and expansion of the area controlled by Isis ... will have huge ramifications for the future stability and freedom of this country and many other countries.
"And we believe the single most important factor that will determine whether or not Iraq overcomes this challenge is political unity."
He added: " As friends of Iraq, we believe the urgent priority must be to form an inclusive government, following on from the recent elections, that can command the support of all Iraqis and work to stop terrorists and their terrible crimes.
"So we hope it will be possible for the government in Iraq to work with moderate leaders among all communities, to work with the Kurdistan Regional Government to resolve long-standing disputes over energy and resources, as well as to address the humanitarian impact of recent events on so many people who are now in urgent need.
"All of these things will help to make an effective security response possible, and we believe that only a united government and real political unity in Iraq will be able to attract the strong practical support from the international community that would be of great help in ensuring the fight against terrorism is won and that their crimes are defeated."
The UK could provide diplomatic support, counter-terrorism cooperation and humanitarian support," he indicated.
"It is also clear to us that we will need new intensified international efforts and mechanisms to stem the flow of arms and fighters to extremist groups, to cut off their finances and prevent them from exploiting the economic assets that they have seized, and ultimately to enable them to be driven out.
"As a permanent member of the UN Security Council, the UK will be making proposals for this work.
"We must also do more together to support Lebanon and Jordan, which are already bearing an immense burden, and must be given support to address potential vulnerabilities to the same Isis threat.
"We will also use the full force of the law to prevent people travelling from Britain for the purposes of committing terrorist acts overseas, and prosecute those who do.
"Isis is a brutal terrorist group whose ideology is alien to this country, and to the Middle East in general. And anyone glorifying, supporting or joining it should understand that they would be assisting a group responsible for kidnapping, torture, executions, rape, and many other hideous crimes.
"They are carrying out these same crimes in Syria and we believe the Assad regime bears a direct responsibility for creating the conditions in which this terrorism can thrive.
"So the UK is stepping up our support for the moderate opposition who are fighting on two fronts - against the regime and against Isis, and we call on other countries to step up that support as well."