(CNN) -- Investigators are searching for an Afghan National Army soldier who shot a rocket at British soldiers in Helmand province, killing three and injuring four others early Tuesday morning, an Afghan Defense Ministry spokesman said.
"We believe these were the actions of a lone individual who has betrayed his ISAF and Afghan comrades," a U.K. military spokesman said. "His whereabouts are currently unknown but we are making strenuous efforts to find him."
In a statement released Tuesday, NATO's International Security Assistance Force said the soldiers were killed at an army patrol base manned by NATO and Afghan troops. The statement said NATO and Afghan officials were investigating the incident.
"We have sacrificed greatly together, and we must ensure that the trust between our forces remains solid in order to defeat our common enemies," Gen. David Petraeus said in a statement.
Petraeus, the top commander of U.S. forces in Afghanistan, said working together was paramount.
"This is a combined, joint mission, Afghan and Alliance troopers fighting shoulder-to-shoulder against the Taliban and other extremists," he said.
Afghan Army Chief of Staff Gen. Sher Mohammad Karimi expressed sympathies to the families of the deceased soldiers, describing them as "coalition partners."
"Our ongoing, partnered investigation will seek to determine how this event could have occurred and we will prosecute those responsible," he said in a statement.
The British Ministry of Defence echoed the mourning, calling the shooting incident a "suspected premeditated attack."
"This is a despicable and cowardly act, and my thoughts go out to the family and friends of those who have lost their lives," said British Defence Secretary Liam Fox. "This incident will be thoroughly investigated by ISAF and the Afghan security forces, and we will do everything we can to bring the individual responsible to justice."
"Training and developing the Afghan National Security Forces is vital to the international mission in Afghanistan and today's events will not undermine the real progress we continue to make. British and ISAF forces are working shoulder to shoulder with Afghans and will continue to do so undeterred," Fox said.
CNN's Matiullah Mati contributed to this report.