NAIROBI (Reuters) - Kenyan President Uhuru Kenyatta accused the judiciary and other agencies of undermining efforts to root out corruption, addressing a campaign pledge from his 2013 election and one which he is likely to be challenged on in next year's vote.

Despite pledges four years ago to crack down on corruption, critics and opponents say the government has been slow to pursue top officials, adding that only top convictions will break what they call a culture of impunity.

The country's anti-corruption commission has been beset by infighting and there are still frequent reports of scams involving ministries. A former chairman of the commission, since pushed out, said in March that around $6 billion, a third of the annual state budget, is lost to graft in Kenya every year.

In a televised gathering of officials and executives, Kenyatta sought to defend his record on tackling corruption, saying he had boosted budget allocations to the Ethics and Anti-Corruption Commission (EACC) and fired ministers and top officials for links to graft. The latter was in 2015, more than two years into his presidency.

Kenyatta also said his efforts were undermined by slow prosecutions by the judiciary and investigative agencies.

"If there is any issue that has frustrated me, it is this issue," he told the meeting, citing a backlog of more than 600 corruption cases pending with the courts.

He also criticized the auditor general for writing reports about misuse of state funds without naming those responsible, and accused the opposition of politicizing an issue that was a matter of national concern, not party politics.

"Corruption is just being used as a political circus," he said.

Kenyatta and his deputy William Ruto have launched the Jubilee party to replace a looser coalition ahead of their re-election August 2017 campaign, when they are expected to face another challenge from veteran rival, Raila Odinga.

(Reporting by Duncan Miriri; Editing by Edmund Blair)