LONDON (Reuters) - Nigerian state oil company Nnpc says Eni's local subsidiary failed to obtain its consent before announcing an agreement to sell onshore oil assets to Oando Plc, a failure that may have violated the terms of a joint operating agreement.

This is according to a letter seen by Reuters, which raises questions about the speed of the deal, announced on Monday, and highlights the difficulties international oil majors have encountered in their multi-year efforts to sell onshore oil and gas assets in Nigeria.

In the letter, dated Sept. 4, Nnpc writes that Eni's subsidiary, Nigerian Agip Oil Company Ltd (Naoc), did not seek its consent before the transaction was announced and that this is mandatory before transferring an interest in a joint venture.

The state oil company's subsidiary Nnpc Exploration and Production Limited (Nepl) holds a 60 percent stake in a Naoc joint venture.

"Eni confirms that there has been no breach of the JV agreement in the sale of Naoc to Oando, and that it fully complies with applicable regulations and agreements. Nnpc has a right of first refusal on the JV shares, but Eni has no contractual obligation to inform Nnpc in advance of the transaction, partly because the information was 'price sensitive' to the potential buyer," an Eni spokesperson commented.

"Speculation about 'breaches' is inappropriate both in terms of content and timing," he added.

Nnpc spokesman Garba Deen Muhammad confirmed that Nepl had sent the letter to Naoc, but specified that the letter did not indicate an objection to the transaction.

"Nepl is only drawing attention to some important clauses in the joint operating agreement, which may have been overlooked by mistake. Compliance with these clauses will protect the transaction now and in the future," he said.

Oando did not comment on the letter, but said he trusts that "as Nepl requested, Naoc will work to ensure that their concerns are taken into account."

Oando also added that Eni has not assigned its 20 percent stake in the Naoc JV to Oando, but has signed an agreement to sell 100 percent of Naoc Ltd's shares, subject to all regulatory and partner approvals and due diligence.

Oil managers say the conclusion of asset sales is critical to spur investment in onshore oil and gas assets, but legal and regulatory problems have stalled other deals, notably the sale of Exxon Mobil assets to local company Seplat.

Nigeria, generally Africa's largest oil exporter, has struggled to extract in recent years due to theft and years of underinvestment. Almost all international oil majors, including Shell and Exxon, have ongoing onshore sales due to theft and oil spills, ongoing clashes with communities, and more targeted exploration budgets.

(Translated by Chiara Scarciglia, editing Sabina Suzzi)