WASHINGTON (AFP) - The United States and Vietnam on Monday opened their first formal military relationship since their war, another sign of growing cooperation amid high tensions between Hanoi and China. The US and Vietnamese militaries signed an agreement in Hanoi setting up cooperation in health, setting the stage for exchanges and research collaboration in military medicine, a US Navy statement said. The former war foes have been steadily building ties and last month held a joint naval drill. But Monday's agreement marks the first formal military cooperation since the normalization of diplomatic relations in 1995, the navy said.