The "deconfliction line" between US European Command and the Russian Ministry of Defense was established earlier this week, Pentagon spokesman John Kirby said.

"I don't have any information about whether it's been used," Kirby told reporters. "It's only been in place for a couple of days."

The line is manned by "staff level officers" from US European Command.

"We know it works," Kirby said. "When we tested it they did pick up the other end and acknowledged that they got the call."

The Pentagon spokesman noted that a "deconfliction line" had been used previously between the United States and Russia to prevent incidents in Syria.

"We want to be able to have a way of speaking directly at an operational level with the Russian Ministry of Defense," Kirby said.

"We think it's valuable to have a direct communication vehicle... to reduce the risks of miscalculation and to be able to communicate in real time if need be," he said.

He said it was particularly important because airspace over Ukraine is "contested by both Russian and Ukrainian aircraft" and the contested airspace "buttresses right up against NATO" countries.