PHNOM PENH, Cambodia -- Cambodian Prime Minister Hun Sen, whose country still grapples with deadly war remnants, on Sunday urged Ukraine not to use cluster bombs, after Washington announced plans to send the weapons to Kyiv to fight Russian troops. Humanitarian groups have strongly condemned the US decision to supply cluster munitions, which can go undetonated and potentially endanger civilians for years to come. "It would be the greatest danger for Ukrainians for many years or up to a hundred years if cluster bombs are used in Russian-occupied areas in the territory of Ukraine," Hun Sen tweeted. He cited Cambodia's "painful experience" of US cluster munitions dropped in the early 1970s, a foreign legacy that has left tens of thousands maimed or killed. "It has been more than half a century. There have been no means to destroy them all yet," Hun Sen added. "As my pity for the Ukrainian people, I appeal to the US president as the supplier and the Ukrainian president as the recipient not to use cluster bombs in the war because the real victims will be Ukrainians," he said. Washington said it had received assurances from Kyiv that it would seek to minimise the risk to civilians, with US President Joe Biden admitting that supplying Ukraine with the weapons was a "difficult decision". The United States dropped millions of bombs on Cambodia and Laos during the Vietnam War in the 1960s and 1970s in an attempt to hit communist bases. And following 30 years of civil war which ended in 1998, Cambodia is among the most heavily mined countries in the world. The effects of the US bombing campaign and minefields left from conflict have long been felt, with around 20,000 Cambodians killed over the last four decades after stepping on landmines or unexploded ordnance. Clearance work continues to this day, with the government vowing to clear all mines and unexploded ordnance by 2025. In January, a group of Ukrainian deminers visited Cambodian minefields to learn from decades of bitter experience.