Washington: North Korea fired unidentified projectiles twice into the sea off its eastern coast on Friday, in its third round of weapons tests in just over a week, South Korea's military said.

The increased testing activity is seen as brinkmanship aimed at increasing pressure on Seoul and Washington over the stalled nuclear negotiations. North Korea also has expressed frustration at planned US-South Korea military exercises, and experts say its weapons displays could intensify in the coming months if progress on the nuclear negotiations isn't made.

South Koreans watch a TV showing a report of a North Korea missile launch in an undated picture. Credit:AP

By test-firing weapons that directly threaten South Korea but not the US mainland or its Pacific territories, North Korea also appears to be dialling up pressure on Seoul and testing how far Washington will tolerate its bellicosity without actually causing the nuclear negotiations to collapse, analysts say.

Seoul's Joint Chiefs of Staff said the launches were conducted at 2.59am and 3.23am from an eastern coastal area but did not immediately confirm how many projectiles were fired or how far they flew. An official from the JCS, who didn't want to be named, citing office rules, said more analysis would be required to determine whether the projectiles were ballistic missiles or rocket artillery.