Violence is likely to increase in Afghanistan this year as foreign security forces expand into new areas and the government steps up its campaign against a booming trade in opium and heroin, a senior US official warned Monday. Richard A. Boucher, the assistant secretary of state for South and Central Asia, said Taliban rebels "certainly have the ability to continue doing what they are doing for a while and be very nasty." "We will probably see a rise in violence this year as NATO spreads into areas in a more dense fashion, as the insurgents try to test the new forces (and) as the government takes on the narcotics traffickers in new areas," he told reporters in Kabul. NATO is gradually assuming control of security in Afghanistan from a US-led coalition. By midyear, NATO troops are set to take over volatile southern regions and by September they are expected to control the entire country.