Q&A: Inter-Korean crisis
Relations between the two Koreas remain fraught following an exchange of artillery fire in November 2010 across the disputed western maritime border that left four South Koreans dead. It was one of the most serious incidents between the two nations since the Korean War ended and came just months after the sinking of a South Korean warship, the Cheonan.
Meanwhile, North Korea has begun transferring power from ailing leader Kim Jong-il to his son, Kim Jong-un - a process that some analysts believe is behind North Korea's recent actions. The BBC looks at the background to the current situation.
What sparked the exchange of fire?
South Korea says North Korean troops began firing artillery towards Yeonpyeong island, a small outpost very close to the disputed Yellow Sea border, at 1424 local time on 23 November. Two soldiers and two civilians were killed and several other people injured. Numerous buildings were destroyed. South Korea returned fire - there is no information on whether casualties were sustained on the North Korean side.
It is not clear what provoked the incident, although the disputed border area has been the scene of numerous clashes in the past. North Korea had protested earlier in the day about South Korean military exercises - which involved firing into the sea - being held near the island.
Why were ties tense between the two Koreas?
An unresolved row over a sunken warship had left relations between the two neighbours - who remain technically at war - at their lowest ebb in many years.
On the evening of 26 March 2010, the Cheonan, a South Korean warship, was sailing off Baengnyeong island close to the disputed maritime border when an explosion split it in two. Fifty-eight sailors managed to escape but 46 were killed.
Investigators looked at whether a mine left over from the Korean War could have been to blame, or an internal malfunction on the ship. But they concluded that what sank the ship was a torpedo fired from a North Korean submarine.
North Korea denies any role in the incident. It has rejected the investigators' findings and demanded it be allowed to conduct its own probe - something which Seoul has ruled out.
How did South Korea react?
The Cheonan incident caused outrage in South Korea. After the investigators' report was released, South Korea suspended trade ties with North Korea and resumed cross-border propaganda broadcasts.
It also referred the issue to the United Nations, seeking some kind of statement condemning the North. But in the end it had to settle for a Security Council statement that condemned the sinking but avoided blaming Pyongyang.
Western diplomats said China had blocked attempts to condemn North Korea, at least partly to avoid any action from its unpredictable ally that could escalate tensions in the region.
What happened next?
In the wake of the watered-down statement, the US began a series of joint military exercises with South Korea, while China continued to call for restraint.
The two Koreas exchanged fire across their land border in October but ties between the two began to improve slightly, amid signs that North Korea wanted to re-engage with its southern neighbour.
Pyongyang offered to resume reunions of divided families and also requested military talks. Seoul, meanwhile, sent rice to North Korea for the first time in two years, to help people in flood-hit areas.
But the shelling once again raised serious tensions between the two sides.
How did nations react to the 23 November shelling?
South Korea bolstered its military capability on Yeonpyeong island and promised air strikes if any further attacks took place. It also has a new defence minister - the previous one stepped down amid domestic criticism that Seoul's response to the incident had been weak.
The US has expressed strong support for South Korea, and the two sides have held more military exercises.
China and Russia have again called for restraint on the peninsula. Beijing wants to convene emergency international talks to reduce tensions, but the US and South Korea have not agreed.
Serious divisions between members of the international community were clear when the UN Security Council met in December to discuss the issue, but failed to reach agreement on the wording of any joint statement.
North and South Korea have continued to trade harsh rhetoric.
Early in 2011 the two Koreas agreed to hold military talks, aimed at paving the way for a higher-level meeting. These broke down in February.
But South Korea has agreed in principle to a North Korean offer to resume talks on humanitarian issues. A date is yet to be set.
What about the nuclear issue?
On this there has been no substantive movement. International talks involving the two Koreas, China, Russia, Japan and the US aimed at ending North Korea's nuclear ambitions remain permanently stalled, despite Chinese calls for them to resume.
The revelation in November that North Korea has a modern uranium enrichment facility with at least 1,000 centrifuges - potentially offering Pyongyang another route to a nuclear weapon - made further talks even less likely. US officials said they were "stunned" at the scale of the facility, although not surprised that it existed.
Satellite images published in February suggest the North may now have completed work on a new launch site for long-range missiles.
But Adm Robert Willard, head of the US Pacific Command, said there were no signs of an imminent launch.
What is motivating North Korea?
Some analysts believe that provocative acts by North Korea are closely linked to the leadership transfer under way in the communist country. Kim Jong-il - who is believed to be in poor health - is thought to be in the process of handing over power to his designated successor, his son Kim Jong-un. In September, North Korea's ruling party held a rare congress in which the younger Kim was given two key roles.
Mark Fitzpatrick, of the International Institute for Strategic Studies, said recent incidents such as the sinking of the Cheonan and the artillery firing at Yeonpyeong island were unlikely to be rogue actions by the North Korean military. Rather, he said, they would be portrayed by North Korea as victories and seen by Kim Jong-il as a way of bolstering the standing of his young son.
Others suggest that a power struggle between North Korean leaders and the military, the greatly-reduced flow of aid from South Korea or frustration with stalled nuclear talks could have contributed to the months of tension.