NATO defense ministers are expected to give the go-ahead for a new U.S. initiative on June 7 that will increase the readiness of the alliance's troops in case of a crisis amid continued concerns about Russian assertiveness in Eastern Europe, alliance chief Jens Stoltenberg says.

The meeting in Brussels will take place just five weeks before a full summit of NATO leaders.

"I expect we will also agree on a NATO Readiness Initiative -- the Four Thirties," Stoltenberg told a news conference in Brussels on June 6.

"NATO’s Readiness Initiative means that allies would, by 2020, have 30 mechanized battalions, 30 air squadrons, and 30 combat vessels ready within 30 days or less."

Thousands of NATO troops are already stationed on standby in the Baltic states and Poland as a deterrent, and Stoltenberg stressed that the Four Thirties are about increased coordination and better mobility.

"This is not about setting up or deploying new forces -- it is about boosting the readiness of existing forces across each and every ally," Stoltenberg said.

"This is about establishing a culture of readiness and we need that because we have a more unpredictable security environment, we have to be prepared for the unforeseen," Stoltenberg said.