A South Korean naval vessel fired warning shots after two North Korean patrol boats crossed the disputed maritime border on Friday, just before US President Barack Obama arrived in Seoul for a two-day visit. The two North Korean boats, which normally serve to keep fishing boats on the right side of the boundary, crossed “one nautical mile south” into South Korean waters, a defence ministry official told reporters. The pre-dawn incursion prompted a South Korean naval ship to fire several warning rounds, after which the two vessels retreated to the North side of the border. The official said the patrol boats might have been chasing Chinese crab-fishing boats operating illegally in the area. “The North might have wanted to check the South’s military vigilance.” South Korean Defence Ministry Official “Or the North might have wanted to check the South’s military vigilance,” he added. The North does not officially recognise the Yellow Sea boundary, which was unilaterally drawn by the US-led United Nations forces after the 1950-53 Korean War. The border has been the scene of brief but bloody naval clashes in 1999, 2002 and 2009. The Korean conflict ended in an armistice instead of a peace treaty and technically the two Koreas are still at war. It is not rare for North Korean patrol boats and fishing boats to cross the unmarked sea border into the South, but the timing was sensitive on Friday because of Obama’s arrival. In other news, Pyongyang on Friday denied accusations from Seoul that it sent small, unarmed drone aircraft over the border recently. Three of the drones crashed and were analysed by the South, which said they came from North Korea. Rodong Sinmun , the official newspaper of the North Korean ruling party, said the claim that the drones came from the North was part of a “heinous conspiratorial farce” aimed at making the North look like it is engaging in belligerent behaviour. It added that Seoul is trying to provoke a war. South Korean officials say they suspect the three drones were flown on possible surveillance missions. North Korea has recently been touting its drone programme, a relatively new addition to its arsenal. It has prominently displayed large drone-like aircraft in military parades. According to the North’s state-run media, leader Kim Jong-un watched a drone attack drill on a simulated South Korean target last year. Tensions are high on the Korean peninsula, with multiple indications that North Korea might be planning to carry out a fourth nuclear test. North Korea has conducted three tests in 2006, 2009 and last year.