South Korea’s spies had information indicating North Korea might attack a front-line island in August, but the intelligence chief dismissed it as a routine threat.

Yeonpyeong Island, a tiny enclave of civilians and military bases located near a disputed maritime border, endured a barrage of North Korean shells last week, and lawmakers in Seoul slammed the government Thursday for the intelligence failure. The surprise revelation came the day before in an unusually candid private briefing by spy chief Won Sei-hoon.

In the wake of the attack — in which two South Korean marines and two civilians died — the defense minister has resigned. President Lee Myung-bak has been criticized for leading a military whose response to the attack was seen as too slow and too weak: The North fired 170 rounds compared with 80 returned by South Korea.

Won told lawmakers that South Korea had intercepted North Korean military communications in August that indicated Pyongyang was preparing to attack Yeonpyeong and other islands in a disputed slice of sea that has often been the focus of North Korean aggression. Won didn’t expect that attack to be on civilian areas and considered it a “routine threat,” according to the office of lawmaker Choi Jae-sung who attended the closed-door session.

“Our intelligence system didn’t work,” Jun Byung-hun, the chief policymaker of the main opposition Democratic Party, said in a statement Thursday.

The National Intelligence Service declined to comment.

“It’s a clear dereliction of duty by the military and intelligence authorities,” the conservative Munhwa Ilbo newspaper said in an editorial Thursday. “It frankly showed the national security system is basically in serious disorder.”

South Korea’s Joint Chiefs of Staff tried to play down Won’s comments, saying the intelligence was that North Korea ordered its troops to prepare to return fire should South Korea conduct artillery drills.

That explanation did nothing to cool anger among opposition lawmakers, many of whom were already critical of Lee’s handling of the attack.

Democratic Party floor leader Park Jie-won said even with the world’s best weapons, the government “cannot guarantee our national security” if intelligence and military authorities aren’t capable of analyzing information.

Meanwhile, about 700 conservative activists rallied against the North, chanting slogans like “Let’s retaliate!” and waving small South Korean national flags at an auditorium in Seoul.

Won also told lawmakers that North Korea is likely to strike again. He said the North probably carried out last week’s attack in part because it needed a “breakthrough” amid internal dissatisfaction over a plan to transfer power from Kim Jong Il to his youngest son, Kim Jong Un, Choi’s office said.

“The artillery attack was part of North Korea’s efforts to boost the status and legitimacy of Kim Jong Un,” said Atsuhito Isozaki, a North Korea expert at Keio University. “He is too young, with no military credentials. But he is a four-star general, and North Korea had to do something to match his nominal title.”

It wasn’t clear how much damage North Korea suffered during last week’s artillery exchange. But satellite photos showed only about 10 of South Korea’s 80 rounds landed near North Korean army barracks along the west coast, according to the office of lawmaker Kwon Young-se who said he saw the images provided Thursday by the National Intelligence Service.

To ease tensions between the Koreas, China has pressed for an emergency meeting in coming days among the six nations who previously negotiated over North Korea’s nuclear program — the two Koreas, China, Russia, Japan and the United States.

Though it walked away from the six-nation talks in April 2009, Pyongyang has since been eager to restart them to gain much-needed fuel oil and aid in exchange for nuclear disarmament. But Washington, Tokyo and Seoul are wary of talking with the North, and their top diplomats planned to meet in Washington on Monday to plot a joint strategy on dealing with North Korea.

Seoul says North Korea must show real commitment to disarm. It noted the North has gone in the wrong direction with its revelation last month of a new uranium enrichment facility that would give North Korea a second way to make nuclear bombs.

Beijing was also hosting senior North Korean officials Thursday for talks about boosting their countries’ ties.

Yeonpyeong Island was quiet Thursday.

Some residents got off the ferry from the mainland, packed bags at their homes, and then rushed back to catch the return ferry. At one home, a woman swept out broken glass and threw away ruined belongings. Her husband covered windows with sheets of plastic.

South Korea lifted a fishing ban on Yeonpyeong on Wednesday and extended the fishing period by one month, to the end of December, for the zone south of the island, according to Seo Kyong-won, an official of the Ministry for Food, Agriculture, Forestry and Fisheries.