Pentagon chief says US will not 'overreact' to N.Korea missile launches

US Defence Secretary Mark Esper has stressed that Washington will not overreact to a series of missiles launches by North Korea in recent weeks.

"The key is to keep the door open for diplomacy... we're not going to overreact to these, but we monitor them, we watch them closely and we're cognizant of what's happening," Esper told reporters travelling with him to Japan.

He added that he would discuss North Korea's recent missile launches with his counterparts in Japan and South Korea during his upcoming visit.

North Korea fired missiles into the sea off its east coast for the fourth time in less than two weeks, as Pyongyang expressed anger at US-South Korean military drills that began on Monday.