A North Korean patrol boat repeatedly crossed the disputed Yellow Sea border with the South in an apparent show of force at the start of South Korea-US military drills, Seoul's defence ministry said Tuesday.

The incursion took place three times overnight Monday and at one point the North Korean naval vessel had reached two nautical miles inside the South side of the border.

No shots were fired and the patrol boat eventually retreated after warnings from the South Korean navy, defence ministry spokesman Kim Min-Seok said.

"We suspect this is aimed at testing our military preparedness", Kim told reporters, saying it was apparent that the vessel had "intentionally violated" the boundary.

North Korean incursions over the maritime border -- which it does not officially recognise -- are not unusual and there were at least three last year.

This was the first such incident in 2014 and it came as South Korea and the United States on Monday launched their annual joint military exercises, which Pyongyang routinely condemns as rehearsals for invasion.

The maritime boundary, which was unilaterally drawn by the US-led United Nations forces after the 1950-53 Korean War, was the scene of brief but bloody naval clashes in 1999, 2002 and 2009.

The war ended in an armistice instead of a peace treaty and technically, the two Koreas are still at war.

The start of the South-US drills overlapped with the first reunion for more than three years of families divided by the Korean War -- an event that has raised hopes of greater North-South cooperation.

Pyongyang had initially insisted that the joint exercises be postponed until after the reunion finished on Tuesday, but Seoul refused and -- in a rare concession -- the North allowed the family gathering to go ahead as scheduled.

The annual "Key Resolve" and "Foal Eagle" drills will last until April 18 and involve a combined total of 12,700 US troops and many more from South Korea.

Last year's drills fuelled an unusually sharp and protracted surge in military tensions, with Pyongyang threatening a pre-emptive nuclear strike, and nuclear-capable US stealth bombers making dummy runs over the Korean peninsula.

US defence officials have indicated -- in an apparent effort to mollify the North -- that this year's drills will be slightly toned down, with no aircraft carrier and no strategic bombers.

However, the South Korean Defence Ministry spokesman insisted Monday there would be "no readjustment" in the scale of the manoeuvres.