Witnesses and an army officer said a suicide bomber threw himself at Nato troops in the main bazaar of Lashkar Gah, the provincial capital, on Thursday.
"The bomber was on foot and hurled himself at the convoy of Nato," said Shamsuddin, an Afghan army officer near the scene.
Yusuf Ahmadi, a spokesperson for the Taliban, claimed responsibility for the attack and told Aljazeera that at least 14 people died in the attack, including seven British soldiers.
A spokeswoman for the British defence ministry confirmed that several British soldiers had been injured but said "We have no report of British fatalities".
British troops make up the bulk of Nato forces in Helmand, a Taliban stronghold and the heart of the illegal opium industry in the world's biggest producer.
Ghulam Muhiddin, the spokesman for Helmand's governor, said the attacker targeted the vehicle of a British aid organisation.