Foreign Secretary Philip Hammond is told the UN that Britain will continue to bomb Islamic targets for "as long as it takes" and urged leaders to unite to fight against the extremist group.
At the UN General Assembly in New York, he said leaders must work together to "crush" IS and "banish its ideology from the face of the earth".
One of four British citizens believed to have joined Islamic State appears to have tweeted mocking the Prime Minister after hearing she has been added to a UN sanctions list.
A post from an account believed to belong to Sally-Anne Jones was published on the social media site Twitter.
Sally-Ann Jones, from Chatham, is one of four individuals that have been put on the list that targets al-Qaeda and Islamic State operatives.
The 45-year-old former punk rocker was married to IS fighter Junaid Hussein, who was killed in US-led coalition air strikes in August.
ITV News cannot independently verify the account.
Britain will continue bombing IS targets, Philip Hammond tells the UN as he urges leaders to unite to "crush" the extremist group.
It made for good headlines, but in a world where UK launches drone strikes on its citizens in Syria, are travel sanctions even a threat?
Government moves to tackle Islamic State recruitment by applying international sanctions to British citizens who joined the terror group.