KATHMANDU (AFP) - The United States warned Nepal on Sunday that political strikes threatening to paralyse the country could lead to its travel warning being reinstated. Opposition leaders called a countrywide shutdown on Saturday - forcing shops to close, crippling transport and sparking violence - after a senior party activist was killed in a prison in the southern district of Chitwan. The opposition NC, which is demanding justice for the killing, has called for a repeat on Monday of the strike, which saw protesters blocking roads, torching cars and vandalising shops across the country. "The US recently lifted the travel warning for American citizens hoping to visit Nepal and we are actively seeking to bring American investors here," Scott DeLisi, the US ambassador to Nepal, said on his Facebook page. "This type of political violence puts our efforts at risk and threatens to recreate the atmosphere that led to the travel warning in the first place." Shiva Poudel, chairman of the NC's youth wing in Chitwan, was critically injured when a group of inmates attacked him on December 6 and he died in Kathmandu on Saturday. He was on remand, having been accused of murdering a political rival. The NC demanded an investigation into his death and called on the Maoist-led government to withdraw cases filed against Poudel, declare him a martyr and compensate his family. Hundreds of police with wooden sticks and riot shields patrolled the streets of Kathmandu on Sunday. Prime Minister Baburam Bhattarai offered Poudel's family one million rupees ($12,000) and asked the party to call off Monday's strike after an emergency cabinet meeting on Saturday. Nepal's civil war ended in 2006 after a decade of fighting that killed some 16,000 people, but the former destination on the legendary hippy trail remained unstable as political leaders struggled to reach a lasting agreement. But political parties last month reached a breakthrough deal that allows for the reintegration of thousands of former Maoist rebel fighters as the parties work to complete a new constitution. The US State Department warned Americans in January of the risk of street violence in Nepal, but cancelled the advisory on December 6 saying it "believes country conditions have changed considerably over the past year".