Official letters from the NGO Board - the government-run body that registers and regulates NGOs - to the Kenya Human Rights Commission (KHRC) and Africa Centre for Open Governance (AfriCOG) said the two organizations risked punishment for administrative and tax reasons.

International and domestic observers have said the election process was largely free and fair, but opposition leader Raila Odinga has disputed the official results, which show incumbent President Uhuru Kenyatta won by a margin of 1.4 million votes.

The NGO Board did not return calls or emails seeking comment on the letters and Reuters reporters were not permitted to enter its offices.

Mwenda Njoka, a spokesman for the interior minister, said the letters, circulating on social media, were genuine. AfriCOG and KNRC said they had not received any official communication.

"This is an attack on any kind of independent voice," said Gladwell Otieno, the executive director of AfriCOG.

Otieno repeatedly raised concerns about what she described as insufficient preparations by the election board in the run-up to last Tuesday's elections, when Kenyans chose a new president, lawmakers and local representatives.

Both organizations also expressed public concern over the unsolved torture and murder of a key election official a week before the vote.

International rights groups Human Rights Watch, Amnesty International and the United Nations High Commissioner for Human Rights Zeid Ra’ad Al Hussein urged the government to respect the work of NGOs.

"The High Commissioner called for civil society actors and media to be allowed to work without hindrance or fear of retaliation," the U.N. said in a statement from Geneva.

POLITICAL TARGETS

Odinga has not yet provided any evidence of rigging. His rejection of results triggered demonstrations and a deadly crackdown by police in his strongholds, including Nairobi slums and the western city of Kisumu.

He was due to give a much-anticipated speech to the nation on Tuesday but postponed it until Wednesday, a spokeswoman for his opposition coalition said.

George Kegoro, the head of KHRC, said his organization was compliant with all laws and was being targeted for political reasons. He denied they had failed to pay taxes, operated "illegal" bank accounts or employed foreigners without work permits.

"If you operate in the kind of environment we do, we have to be compliant. The rules are a drag but we observe them,” he said.

His organization had already successfully defended itself in High Court against the same accusations, he said, making the new letter threatening de-registration "a travesty of justice".

"We think its got to do with the politics of the season. We’ve played a leadership role in organizing civil society participation in this election. They (the government) don’t like that."

Otieno said her organization did not fall under rules governing non-governmental organizations and was properly registered.

Njoka denied the organizations were being politically targeted and said: "There were some issues with their auditing and accounting ... If they give good accounts they may not be de-registered."