FACTBOX: Five facts about Afghanistan's Helmand province

(Reuters) - Thousands of U.S. Marines stormed deep into Taliban territory in opium poppy-producing areas of the Helmand River valley on Thursday at the start of a major new offensive Washington hopes will turn the tide of the war in Afghanistan.

Following are five facts about Helmand province.

* Helmand is Afghanistan's largest province, about 60,000 sq km (23,000 miles), making it slightly smaller than the Republic of Ireland. The U.S. Marines join some 9,000 British troops who have been working under Taskforce Helmand since 2006 and are based near the provincial capital, Lashkar Gah.

Canadian, Dutch and other NATO troops have been fighting alongside the British troops in Helmand but U.S. military commanders have described the combat situation in the past year as a stalemate. Existing force levels have not been able to cope with the size and difficulty of the terrain, which includes wide deserts in the south and mountains in the north. In May, the deputy commander of NATO-led forces in the south warned of "a bloody summer ahead."

* Helmand's population is mainly made up of ethnic Pashtuns, Afghanistan's largest ethnic group who have also traditionally been the country's power brokers. It borders Pakistan to its south, Kandahar province to its east and Nimroz province to the west, all mainly Pashtun provinces and heavily influenced by the Taliban. Provincial officials estimate that four out of Helmand's 13 districts are under Taliban control.

* Helmand produces more than half of the opium cultivated in Afghanistan, the source of about 90 percent of the global supply, according to the United Nations Office on Drugs and Crime. In 2008 more than 103,000 hectares of poppy were cultivated. The drug crop is closely tied to the insurgency and the Taliban are mainly funded by the opium trade.

But NATO forces in Afghanistan are not permitted to engage in crop eradication, a policy which limited British tactics in crippling the insurgency. Britain, the United States and other NATO allies have started a number of civilian programmes to offer farmers alternative crops to opium, such as wheat, but Helmand remains Afghanistan's biggest poppy-producing province.

* Helmand is mostly desert, with agricultural fields cultivating opium poppy and food crops, concentrated around the Helmand River, Afghanistan's longest and which cuts through the center of the province.

Most of the province's population is clustered around the river in north and central Helmand, where British and U.S. troops are also mainly deployed.

* U.S. troops have been deployed to Helmand before to bolster British efforts. In late 2006, months after arriving in the district of Musa Qala, British troops were forced to pull out because of daily Taliban attacks that at times reached their perimeter defenses.

The Taliban seized the town in February 2007 and set up a shadow administration. But 10 months later thousands of British and U.S. troops launched an offensive around the district, paving the way for Afghan soldiers to capture the town.

(Sources: British Ministry of Defense, International Security Assistance Force, Reuters reports, United Nations Office on Drugs and Crime)