North Korea fired short-range missiles into the sea off its eastern coast for the second time in a week Monday, prompting a warning from South Korea of "reckless provocation", AFP reports.
The missile tests have clearly been timed to coincide with annual South Korea-US military exercises which kicked off a week ago and run until mid-April.
Two missiles were fired Monday and both flew around 500 kilometers (310 miles) into the Sea of Japan, according to South Korea's Defence Ministry.
Four short-range Scud missiles were fired in similar fashion on Thursday.
Both tests were condemned by Seoul, which urged the North to cease all testing immediately and said it would consider calling for sanctions.
"The North is taking a double-faced stance by making conciliatory gestures on one hand and pushing ahead with reckless provocation on the other," said Defence Ministry spokesman Kim Min-Seok.
The Scuds are at the longer edge of the short-range spectrum, with an estimated reach of 300-800 kilometres (185-500 miles) -- capable of striking any target in the South.
It is not unusual for North Korea to carry out such tests, which often go unreported by South Korea.
But Kim said the Scud firings were of particular concern.
"We believe that the North is testing various ballistic missiles with various ranges as a show of force to threaten us," he said.
Washington initially played down Thursday's firings, but later suggested they violated UN sanctions imposed on the North's missile programme.
UN Security Council resolutions prohibit North Korea "from launching any ballistic missile, and this includes any Scud missile", Pentagon spokesman Colonel Steven Warren said Friday.
Pyongyang routinely condemns the South-US joint exercises as rehearsals for invasion.
Last year they coincided with a sharp and unusually protracted surge in military tensions, that saw North Korea issuing apocalyptic threats of pre-emptive nuclear strikes.
By contrast, this year's drills began as relations between Seoul and Pyongyang were enjoying something of a thaw.
They overlapped with the end of the first reunion for more than three years of families divided by the Korean War -- an event that raised hopes of greater cross-border cooperation.
Pyongyang had initially insisted that the joint exercises be postponed until after the reunions finished. But Seoul refused and -- in a rare concession -- the North allowed the family gatherings on its territory to go ahead as scheduled.
Most analysts believe the missile tests reflect Pyongyang's need to flex its muscles in the wake of the reunion compromise.
Last week also saw an incursion by a North Korean patrol boat across the disputed Yellow Sea border that has been the scene of brief but bloody naval clashes in the past.
No shots were fired and the vessel retreated to its side of the boundary after repeated warnings from the South Korean navy.
North Korea has hundreds of short-range missiles and has developed and tested -- with limited success -- several intermediate-range models.
Its claims to have a working inter-continental ballistic missile have been treated with scepticism by most experts, but there is no doubt that it is pushing ahead with an active, ambitious missile development programme.