Defence ministers from the US, Japan and South Korea met in Singapore at the weekend to weigh punitive steps against North Korea as the UN Security Council prepared to take up the crisis triggered by the sinking of the Cheonan.
Robert Gates, the US defence secretary, told his counterparts on Saturday that "it's important we have a unified front to deter further provocations", according to Geoff Morrell, his press secretary.
But in a BBC interview broadcast on Sunday Gates said there were few, if any, effective options against North Korea, short of military action.
"As long as the regime doesn't care what the outside world thinks of it, as long as it doesn't care about the well-being of its people, there's not a lot you can do about it, to be quite frank, unless you're willing at some point to use military force," said Gates in the interview.
"And nobody wants to do that."
He also warned that given North Korea's volatile reputation, it is "possible there are other provocations to come".
On Sunday Lee joined more than 5,000 people including legislators, government officials and relatives of the 46 dead sailors at Seoul National Cemetery, making little mention of the hostilities between North and South Korea in his speech.
"We still have an unfulfilled dream," he said.
"It is to become a unified country living in freedom, peace and prosperity with North Korean compatriots, who are still suffering from poverty and suppression."
On Saturday, Lee tried to lower fears of an armed conflict, saying there's no possibility of a full-scale war on the Korean peninsula.
|Families of the 46 dead sailors joined Lee Myung-bak, centre, at the memorial [Reuters]
South Korea officially referred the North to the Security Council late last week over the sinking, taking its strongest step yet in its efforts to hold the North to account.
The US and South Korea, while condemning the North's alleged torpedo attack, have called for calm and avoided talk of a military response against Pyongyang.
But North Korea has repeatedly denied responsibility for the sinking of the warship and warned of retaliation if it is hauled before the UN Security Council.
Tensions have been running high on the Korean peninsula since a multinational probe concluded last month that a North Korean torpedo sank the Cheonan, triggering trade reprisals from South Korea and threats of war from the North.
Earlier the US announced joint naval exercises with South Korea next week "to ensure readiness and to deter future aggression" by North Korea, but Gates said the anti-submarine drill may be delayed pending a UN response.