Members of the elite 45 Commando Royal Marines were sent into the area several days ago but their presence there was not made public until Tuesday.
The Marines, skilled in mountain warfare, are operating at elevations above 9,500 feet, Royal Marines spokesman Lt. Col. Paul Harradine said. He would not specify where the Marines were deployed, but it was believed they were in rugged terrain south of the capital near the Pakistani border.
U.S. military spokesman Maj. Bryan Hilferty would not say how many American troops were in the area following up on last month's Operation Anaconda - a 12-day assault on Taliban and al Qaeda forces in the eastern Shah-e-Kot mountains.
At the Pentagon briefing, Air Force Brig. Gen. John Rosa said the mission, which the British call Operation Ptarmigan after a bird that changes color to blend into its environment, began Monday.
Rosa said the mission was part of the ongoing effort to destroy al Qaeda and Taliban remnants in southeastern Afghanistan - which the U.S. military refers to as Operation Mountain Lion.
There were no details of the number of U.S. and Afghan forces involved or exactly where they were, in an operation U.S. military spokesman Major Bryan Hilferty described as the first large-scale coalition combat operation since the two-week long Operation Anaconda was launched in early March.
CBS News Correspondent Lee Cowan reports that is unclear how many al Qaeda or Taliban fighters are dug in, but the amount of firepower British Marines are choppering to the site indicate they're prepared for a fight that could last for weeks.
The battlefield is even less hospitable than what U.S. forces endured during Anaconda - higher and deeper into the Afghan mountains - in conditions that are hardly improving, despite the onset of spring.
Harradine said coalition forces had suffered no casualties so far in the operation but would not say whether there had been any clashes with al Qaeda or the Taliban.
They would also search for and destroy ammunition, caves and any other places that could be used as future al Qaeda or Taliban bases, he said at Bagram, an air base just north of Kabul which U.S.-led forces are using as their main staging post.
"This is a very specific mission, of which there will be more, to achieve that overall broad objective of dealing with residual Taliban and al Qaeda forces which remain in Afghanistan," Adam Ingram, Britain's minister for the armed forces, told the BBC.
Ingram said that capturing al Qaeda leader Osama bin Laden remains a prime objective, although his whereabouts are unknown.
"We've got to operate on the basis that he's still alive, that he may well still be in the country, and if we do obtain knowledge of where he is then clearly we will pursue him with all vigor," Ingram said.
Brigadier General Roger Lane, commander of the British Marines in Afghanistan, said it was clear there was much work for his men to do.
"We stand shoulder to shoulder with the United States and our other coalition allies in the global war on terrorism and that's exactly what we are doing," he said.
The Royal Marines have not been involved in a major combat operation since the Falklands War against Argentina 20 years ago.
Despite on the ongoing military operations, Afghanistan's interim ruler, Hamid Karzai, flew Tuesday to Rome to escort the country's former king home after 29 years in exile. The former monarch, Mohammad Zaher Shah, is expected in Kabul early Thursday.
Zaher Shah's return is widely seen as a major step toward national reconciliation after a generation of armed conflict. He was to have come home last month but his return was postponed due to security concerns.
A special police unit will coordinate 24-hour protection for the 87-year-old former ruler, with 50 officers stationed inside his house and 100 more taking up positions outside. Security forces have already blocked off the only three streets leading to the residence with barbed wire and concrete barriers.
Zaher Shah has lived in Italy since he was ousted by his cousin Mohammed Daoud Khan in 1973. In June, the former king is to convene a loya jirga, or grand council, which will choose a new government to serve until elections can be held in 18 months.
"His majesty doesn't have any concerns. He is going to see his children, his countrymen and women," his spokesman Hamid Sidiq said. "He doesn't have any fears in this regard."
In Kandahar, U.S. officials said all personnel had been accounted for after confiscated Taliban weapons exploded accidentally at a demolition range Monday outside of the city.
Four soldiers were killed and a fifth was injured, but Pentagon officials feared the casualty toll could rise because some personnel were missing. The Pentagon identified the dead as Staff Sgt. Brian T. Craig, 27, of Texas; Staff Sgt. Justin J. Galewski, 28, of Kansas; Sgt. Jamie O. Maugans, 27, of Kansas and Sgt. 1st Class Daniel A. Romero, 30, of Colorado. Their hometowns were not provided.
Craig, Galewski and Maugans were members of the 710th Explosive Ordnance Detachment based at San Diego. Romero was with the 19th Special Forces Group based at Pueblo, Colorado.
In Kabul, the United Nations announced that over the last seven weeks, a quarter million Afghans have come home from refugee camps in Pakistan, Iran and Tajikistan.
However, U.N. spokesman Yusuf Hassan said millions remain outside the country, waiting to see if conditions stabilize before coming home.
Returning refugees are having trouble finding places to live, especially in the capital, Kabul, where there is a severe housing crisis. In the countryside, many refugees are arriving to find their houses burned or bombed.
Hassan said 40 percent of those returning are moving into urban centers. Kabul alone has seen 80,000 refugees return, and there is little place for them to live.