American and coalition forces have been battling Afghan rebels in the south of the country, close to the border with Pakistan. American war planes have bombed rebel fighters in the mountainous region near the town of Spin Boldak, in what US military officials say is the largest-scale fighting for nine months. Some 200 US special forces troops are engaged in the mountain battle and more are on their way, US Army spokesman Major Robert Hepner told the Associated Press. Another US spokesman said the fighting forces were aligned to one of the Afghan leaders, Gulbuddin Hekmatyar. Up to 18 rebel fighters have been killed, spokesman Colonel Roger King said. \"It\'s the largest concentration of enemy forces since Operation Anaconda,\" Colonel King said - a reference to the US military campaign in eastern Afghanistan last March. Colonel King said the latest battle might last some time, because of the large area involved. \"It\'s rough terrain. There are some caves, there may be more that we don\'t know about, so it could take a considerable period of time,\" he said. The fighting has been centered on rocky, unpopulated terrain around the Adi Ghar mountain, near the border with Pakistan, where as many as 80 fighters were hidden, he said. The fighting was triggered by a small gun battle nearby between US Special Forces and armed attackers as the Americans and Afghan Government troops were working to clear a compound. The allied forces surrounded the group - which locals said had been threatening communities east of Spin Boldak - killing one man, wounding another and detaining at least one other. Apache helicopters sent to investigate came under fire, prompting the deployment of American B-1 bombers, F-16s and AC-130 gunships. (BBC)