South Korea has responded strongly to what it said was a violation of its airspace by a North Korean fighter jet early on Thursday.
The jet crossed a maritime border unrecognised by the North
The South scrambled two fighters to intercept the North Korean MiG-19 and put an anti-aircraft missile base on battle alert, the Defence Ministry in Seoul said.
The two-minute border crossing was the first aerial incursion by the North for 20 years. South Korea called it a provocation and said it would lodge a strong protest.
The incident came hours after the United States announced that Secretary of State Colin Powell would attend the inauguration of President-elect Roh Moo-hyun next week.
Mr Powell is to visit South Korea, Japan and China from the weekend. The stand-off over North Korea's nuclear ambitions is expected to dominate his agenda.
We will strongly protest against the intrusion after analysing their intention
Joint Chiefs of Staff
The BBC's Charles Scanlon in the region says North Korea may be intentionally increasing tension in anticipation of Mr Powell's trip.
The South Korean Joint Chiefs of Staff said Seoul would respond forcefully to Thursday's incident.
"We will strongly protest against the intrusion after analysing their intention," the Joint Chiefs said in a statement.
The fighter crossed the Northern Limit Line near Yonbyong Island in the Yellow Sea - a maritime boundary that Pyongyang does not accept.
Northern Limit Line
Declared by UN in 1953
Not recognised by North
Regularly breached by North's navy
North Korea threatened this week to withdraw from the armistice if economic sanctions were imposed in response to its nuclear programme.
The incursion took place near the site where a South Korean gunboat was sunk in a clash with the North Koreans last June.
Six South Koreans died in the exchange. About a dozen North Koreans are believed to have died.
There were also naval confrontations in the area in 1999.
Four more South Korean aircraft were later sent to the area of Thursday's incursion.