The meeting is being held at the request of Russia and China (Picture: Xinhua/PA).
The United Nations Security Council has been meeting to discuss the threat Russia and China say is posed by US medium-range missiles.
The US tested a ground-launched cruise missile off the coast of California, which hit its target after travelling 500 kilometres.
It followed the United States' withdrawal from the Intermediate-range Nuclear Forces arms control treaty signed during the Cold War.
The US has indicated that the test was intended to send a message to China, which has many intermediate-range missiles and wasn't party to the treaty.
The missile was launched from the US Navy-controlled San Nicolas Island off the coast of Los Angeles, California.
Russian President Vladimir Putin has ordered his country's military to work out a "symmetrical response".
Watch: The US tested its ground-launched cruise missile on Sunday.
Addressing the council, US Ambassador to the UN, Jonathan Cohen, explained the reason the US had withdrawn from the treaty.
"We're here today because the Russian Federation preferred a world in which the United States continued to fulfil its INF Treaty obligations, while the Russian Federation did not.
"Indeed, the Russian Federation and China would still like a world where the United States exercises self-restraint while they continue their arms build-ups - unabated and unabashed."
In addition to the land variant of the Tomahawk cruise missile, the Pentagon has said it also intends to begin testing an INF-range ballistic missile with a range of roughly 3,000 to 4,000 kilometres (1,864 to 2,485 miles).
Both missiles are to be non-nuclear.
US Defense Secretary Mark Esper said this month that he hopes the Pentagon can develop and deploy INF-range missiles "sooner rather than later", but no specific timeline has been announced.