South Korean President Lee Myung-bak apologized to the nation on Tuesday for what he called heartbreaking corruption cases allegedly involving his elder brother and close aides.

"I bow my head and apologize for causing concern to the people due to these incidents," a solemn Lee said in brief televised remarks. "It breaks my heart... that such regrettable things have happened among people so close to me."

The incidents have tarnished the conservative leader's image in the last year of his five-year term, although he has not been personally implicated in them.

A presidential election will be held in December, but Lee is constitutionally barred from seeking a second term.

Analysts said the graft cases would have only a limited effect on the electoral fortunes of his New Frontier Party.

"This won't have any significant impact on the election, because the party has distanced itself from the president, as if they are not related to each other at all," Professor Lee Jun-han at Incheon University told AFP.

Lee's elder brother Lee Sang-deuk was arrested and detained earlier this month pending a corruption trial.

Prosecutors allege the 76-year-old former lawmaker took 600 million won ($525,000) from chairmen of two troubled savings banks between 2007 and 2011 in return for helping them avoid audits and punishment.

Angry bank depositors had hurled eggs at him when he arrived at court the previous day for questioning.

Lee Sang-deuk was seen as the main contributor to his brother's election victory in 2007, playing the role of a troubleshooter behind the scenes.

But critics said he tried to wield too much influence over state affairs once his brother came to power.

Several of the president's closest aides, including former top communications official Choi See-jung, former vice culture minister Shin Jae-min and ex-knowledge economy vice minister Park Young-joon, have also been arrested on graft charges.

Lee's three immediate presidential predecessors - Roh Moo-hyun, Kim Dae-jung and Kim Young-sam - were also tarnished by the wrongdoing of close relatives.

In May 2009, Roh committed suicide by throwing himself off a cliff after he was questioned as a suspect in a multi-million dollar corruption case.

Roh apologized for his family's involvement but did not admit personal wrongdoing.

Lee has made several previous public apologies during his time in office.

"He took a step forward because the apology was needed for the party, for himself and for the people," said Professor Lee.

"However, he will need a bigger apology if the investigation (into his brother) turns out to be true.

"It would have been better if he had urged that the investigation proceed transparently, on top of the apology he made."