WASHINGTON (AFP) - US President George W Bush said Tuesday he was "troubled" by the movement of extremists from Pakistan to Afghanistan and vowed to press Pakistan's prime minister, due here on July 28, on battling extremists blamed for bloody cross-border attacks and crushing a resurgent Al-Qaeda along the frontier. Speaking at a White House Press conference, Bush also said the United States would investigate charges by Afghan President Hamid Karzai that elements of the Pakistani intelligence services had been involved in attacks in Afghanistan. "We'll investigate his charge and we'll work with his service to get to the bottom of his allegation," the US President said. "No question, however, that some extremists are coming out of parts of Pakistan into Afghanistan," Bush said. "And that's troubling to us, it's troubling to Afghanistan, and it should be troubling to Pakistan. "I certainly hope that the government understands the dangers of extremists moving in their country," the US President said. "I think they do." "As a matter of fact, we'll have an opportunity to explore that further" with Pakistani Prime Minister Yousuf Raza Gilani, Bush said. "Pakistan is an ally. Pakistan is a friend," he said. "And I repeat, all three countries - the United States, Pakistan and Afghanistan - share a common enemy." Bush also said the United States is "surging troops in Afghanistan," where nine US soldiers were killed on Sunday in one of the deadliest attacks by the Taliban since the group's ouster by US-backed Afghan forces in late 2001. "We're committed," Bush said, describing the wars in Afghanistan and Iraq as the "big challenge of the time." "You know, the question really facing the country is will we have the patience and the determination to succeed in these very difficult theatres," he said. The US President said he rejects any "artificial" timetable for withdrawing US troops from Iraq and reiterated that the decision must be made according to conditions on the ground. "There's the temptation to let the politics at home get in the way with the considered judgment of the commanders. That's why I strongly rejected an artificial timetable of withdrawal. It's kind of like an arbitrary thing, you know," Bush said. "The Iraqis have invited us to be there. But they share a goal with us, which is to get our combat troops out. As conditions permit." Bush added that "we are succeeding" in Iraq, and "our troops are coming home, based on success." On Sudan, Bush said he wanted to see how an international prosecutor's arrest warrant for Sudanese President Omar al-Beshir "plays out," but that Khartoum could face more sanctions. He said since the United States is not a party to the International Criminal Court, which issued the arrest warrant on war crimes charges this week, Washington would "see how that plays out," he told a news conference.