British Prime Minister David Cameron is seeking Parliament's approval for strikes on the Islamic State (IS) militant group.
Cameron, who has recalled Parliament from break, told legislators in his opening address that IS had shown "staggering brutality."
Cameron also warned that the fight against IS "could take years."
Cameron has been hesitant about seeking Parliament's approval for this recent campaign, after it rejected British air strikes last year against Syrian government forces.
Parties from across Britain's political spectrum have indicated they would support the prime minister for strikes on IS.
The beheading of two U.S. journalists and a British aid worker by an IS militant with a British accent have galvanized public opinion against group, which controls a patchwork of areas in northeastern Syria and northwestern Iraq.
Britain says some 500 of its citizens have gone to fight in Syria and northern Iraq, raising concerns some could return and carry out terrorist attacks in the United Kingdom.
In Copenhagen, Denmark said it will contribute seven F-16 fighter planes to help combat IS militants in Iraq.
Danish Prime Minister Helle Thorning-Schmidt said on September 26 that the planes would not take part in attacks in Syria.
Meanwhile, the European Union's counterterrorism coordinator has said that more than 3,000 Europeans have joined Islamist fighters in Syria.
Gilles de Kerchove told the BBC on September 26 that U.S.-led air strikes would increase the risk of revenge attacks in Europe.
U.S.-led forces have launched nearly 200 air strikes against Islamic State forces in Iraq since August and on September 22 began targeting the group in Syria.
On September 25, activists said U.S.-led air strikes targeted Syrian oil installations held by the group overnight and early on September 25, killing nearly 20 people.
IS has seized large parts of Iraq and Syria in recent months.
De Kerchove said the number of 3,000 included all those who have been to the region, including those who have returned and those who have been killed there.
Earlier this year, experts put the figure at about 2,000.
On September 24, de Kerchove told the European Parliament in Brussels that there was a real risk some of those who went to Iraq and Syria to fight alongside the Islamic State and other militant networks could return and bring violence back to Europe.
With reporting by Reuters, AP, and BBC