(Corrects fund manager's name in 18th paragraph to Akino (not Ando)
* Abe apologises for resignation of defence minister
* Foreign Minister Kishida to add defence portfolio
* Abe promises govt on alert for security
* Talk of early general election emerges
By Linda Sieg and Elaine Lies
TOKYO, July 28 (Reuters) - Embattled Japanese Defence Minister Tomomi Inada on Friday said she was resigning, after a series of gaffes, missteps and a cover-up at her ministry that have contributed to a sharp plunge in public support for Prime Minister Shinzo Abe.
Inada, 58, an Abe protege who shares his conservative views and had been suggested as a possible future premier, had already expected to be replaced in a likely cabinet reshuffle next week that Abe hopes will help rebuild his ratings.
Support for the prime minister has sunk below 30 percent in some polls, due to scandals over suspected cronyism and a view among many voters that he and his aides took them for granted.
Abe apologised "to the people from my heart", in comments to reporters carried live on national television after Inada announced her resignation.
He said Foreign Minister Fumio Kishida would add the defence portfolio to his duties, to eliminate any gap at a time when Japan faces tough security challenges, such as a volatile North Korea.
Abe, however, had drawn fire from both ruling and opposition party lawmakers for retaining Inada despite her missteps and perceived incompetence.
"He should have thrown Inada under the bus long ago … doing so on the eve of a cabinet reshuffle only looks like desperation," said Jeffrey Kingston, director of Asian Studies at Temple University Japan.
The resignation coincided with a report of an investigation into suspicions that defence ministry officials tried to hide logs showing worsening security in South Sudan, where Japanese troops joined in a U.S.-led peacekeeping operation.
Critics said troop deployment in the dangerous environment violated conditions set for such activities in line with Japan's pacifist constitution. No Japanese troops have died in combat since World War Two and the growing chaos in South Sudan fuelled concern.
OPPOSITION DISARRAY, ELECTION CHATTER
The report on the cover-up concluded that laws, including one on public information disclosure, had been violated but did not confirm a direct role for Inada in concealing the logs, a contention she has repeatedly denied.
However, Inada said, the affair had hurt the credibility of the defence ministry and Self-Defense Forces (SDF), as Japan's military is known.
"As the minister in charge of the defence ministry and SDF, I feel my responsibility keenly," she told a televised news conference.