Tensions have escalated on the Korean peninsula over the sinking of a South Korean warship near its maritime border with the North on 26 March.
An international panel says a torpedo fired from a North Korean submarine sent the ship down - but Pyongyang denies this. South Korea has suspended trade ties with the North.
Readers in South Korea discuss the seriousness of the incident and the best way to tackle North Korea.
Sungsoo Ji, teacher, Masan
I've never seen the situation this bad - it's very tense.
I've done my national service and I've still got friends in the army. They're saying that soldiers are getting more training and that things are getting serious.
Our president could be overreacting though - North Korea doesn't want war.
To be honest, I'm not sure I trust the information given to us about the Cheonan sinking. It could be a trick because it's election period at the moment, so it could be some kind of strategy.
If North Korea did sink the ship, it's a really serious situation.
I'm thirty-one and young people like me are worried. But old people who experienced the war worry about it even more because they've lived it.
Kyungho Rhee, student, Seoul
North Korea is trying to send a message. They do this every once in a while causing great anger among South Korean people.
South Korea has been very strong - making it clear where we stand.
Stopping trade with the North is the right thing to do. But I don't think it will bring major results.
There is not much appetite for war here. Our economy is getting stronger and a war would hurt us badly.
We don't have a very strong military. We do have the backing of the US but we can't rely on them. So stopping trade with the North is the best we can do.
As for the long run - it is China who I think can have the greatest impact. They are the key to how this unfolds.
I believe that South Korea and the US have to make an effort to back China into a position where they take action on North Korea. But it is going to be a struggle.
Junwhan, 38, government official, Seoul
The sinking of the Cheonan is a very serious incident. I was shocked and extremely sad for the the death of our sailors.
I am very worried because it's North Korea's most provocative act since the Korean war.
The nuclear test and the long-range missile tests were also very threatening, but they didn't actually hurt Korean people. The latest incident did cost the lives of people.
Several years ago a North Korean ship transgressed the Northern Limit Line [inter-Korean maritime border] and there was a conflict between the two Koreas. The North promised to revenge.
North Korea also wanted to test South Korea's diplomatic power, particularly in its relationship with the US. Also, they know that Korean society is very divided when it comes to how the country should approach the North.
I personally support the more conservative, tough stance against North Korea. I think we should block them severely, so I am all in favour of trade cuts.
The broadcasts are also important, because while Kim Jong-il is our enemy, the people of North Korea are our brothers. They have the right to know what happens in the outside world.
Lee Jae-youn, intern in a company, Seoul
It's a really sad situation. People are worried because the security problem is serious. But some of us have questions about this announcement by our government.
First of all, why has this happened now? We have an election in one week and many acts of provocation by North Korea have occurred in the period before an election. So we wonder - has this warship sinking accident been abused by the ruling party?
The evidence is not clear, yet our government takes the result of the investigation as a fact. But I wonder how the mark of the ink pen still exists [on the torpedo fragment] even after the explosion? And why has North Korea put a signature at the bottom of the torpedo?
Anyway, it's a great sadness for our country. We cried together during the ceremony for the 46 sailors who died. So I hope that this accident is never abused for someone else's benefit.
This is a serious situation, many people are talking about war. The 60th anniversary of the Korean war is coming up and we feel uncomfortable with our president saying again and again that we will revenge them.
Gyuhang Kim, student, Seoul
I and many others suspect the South Korean government of deliberately accusing North Korea, even of making up the proof.
We are well aware of the anti-North Korean sentiment of the government and do not trust the official report at all.
The mark "number 1" on the front part of the torpedo cannot survive such a blow. Some news sites doubting the reliability of the proof have been shut down for no reason and we suspect government censorship.
I also think the government is ruining years of hard work of former presidents, especially Kim Dae-jung, who have worked so hard to take a first step towards reunification.
Not only has President Lee Myung-bak destroyed years of efforts for reunification but he is also endangering South Korea's economy and civilians by risking a war with the North - the prelude to a nuclear or third world war.
Rumours are spreading about the cause of the sinking of the ship - such as a possible mistake with an American submarine during a joint exercise, which was covered up by the South Korean government in order to discredit North Korea.
Choi Jae Woo, 24, science teacher, Cheonan
I am very sad about this incident. When I was in the navy doing my military service, I was working on the same kind of ship. I have never been worried about such an attack.
I am worried about a possible war with North Korea. My friends are also worried about it.
We are really angry about what North Korea did but we don't want a war.
There's lots of arguing going on right now between the conservatives and the liberals. I am conservative.
I say cut them off, stop the trading and prevent their ships from entering our waters. But I wouldn't go further than that as it could get really dangerous.
We can't do much more. The country that has a bigger influence over North Korea is China. I hope China will manage to control them, but I don't expect a change in their stance.