Beirut - The Islamic State group is "completely besieged" in its last major stronghold in Syria's Aleppo province, a monitor says as pro-regime forces pressure the jihadists on several fronts.ISIS fighters were cut off in Al-Bab after forces loyal to President Bashar al-Assad's government severed a road into the northern town, the Syrian Observatory for Human Rights reported."Al-Bab is now completely besieged by the regime from the south and the Turkish forces and rebels from the east, north and west," the monitor said.Regime forcesThe reported siege came as President Donald Trump on Monday pledged that America and its allies would defeat the "forces of death" and keep radical jihadists from gaining a foothold on US soil."We deliver a message in one very unified voice to these forces of death and destruction - America and its allies will defeat you," Trump said as he visited US Central Command, which plays a key role in the US-led mission to fight ISIS in Iraq and Syria."We will defeat radical Islamic terrorism. And we will not allow it to take root in our country," he said.Observatory chief Rami Abdel Rahman said Syrian regime forces and allied militias captured the last route between Al-Bab and Raqa, the jihadists' de facto capital in the country.At least 11 pro-government fighters were killed in ongoing clashes around Al-Bab on Monday, he added.Just 25km south of the Turkish border, the town is seen as a prize by nearly all sides in the complex war.Opposing sidesSince December, Turkey-backed rebels known as the Euphrates Shield alliance have edged towards Al-Bab from the north."Al-Bab is more important for the Turks, who defined the town as a priority for their Euphrates Shield alliance," said Syria expert Thomas Pierret.But the regime was eyeing territory around Al-Bab "to protect the eastern flank of Aleppo city" it recaptured in December, he said.It also sought to use the town as a buffer, "preventing the Euphrates Shield from advancing south" deeper into Syria.Turkey and Russia back opposing sides in the war, but have joined forces in recent months to try to end the conflict.They carried out their first joint bombing raids around Al-Bab in January after brokering a fragile ceasefire between rebels and regime forces.Latest attemptAlong with Iran, they organised a summit in Kazakhstan to try to reinforce that truce ahead of peace talks in Geneva later this month.Those negotiations, hosted by UN special envoy for Syria Staffan de Mistura, are the latest attempt to end the six-year conflict.Several rounds of indirect talks in 2016 between the regime and the opposition, represented by the High Negotiations Committee, ended without notable progress.