JOHANNESBURG, May 14 (Reuters) - The boss of Anglo American said on Tuesday a bid by rival BHP to take over the company forced him to accelerate plans for a spin-off of its South African platinum assets, which come on the cusp of a national election.

While Anglo was already working on its own review of assets including the platinum and diamonds businesses, the timeline had to be speeded up after BHP's approach, Anglo CEO Duncan Wanblad said.

As a pre-condition for its bid, rejected twice by Anglo, BHP had requested that Anglo exit its platinum and iron ore units in South Africa, drastically reducing its presence in the country.

After arguing that the BHP proposal undervalued Anglo, the London-listed mining giant laid out a strategy on Tuesday that includes a potential break-up of the group by demerging or selling its steelmaking coal, nickel, diamonds and platinum businesses.

That includes a demerger of its Johannesburg-listed Anglo American Platinum unit, known as Amplats. The plan sent Amplats's shares down as much as 10%.

"The only thing that the BHP approach did is that it forced the timeline on work that we were already doing," Wanblad said on a conference call after Tuesday's announcement.

"This is an acceleration of a strategy process that we were already executing. I have to say that I would probably not have announced it at this particular point of time," he added, referring to the looming election in South Africa on May 29.

South Africa Mines Minister Gwede Mantashe told Reuters he had no issues with the proposed demerger of Amplats, even if the ultimate fate of the business remains unclear.

"It is... important for Anglo to restructure itself to get optimal performance of every portfolio in their stable," Mantashe said. "And I hope it will work for them."

Mantashe, who had opposed BHP's bid, said he hoped Anglo would continue resisting the offer from its rival, which was raised to $43 billion on May 7.

Shares in Anglo unit Kumba Iron Ore, which under the review laid out on Tuesday would remain within the group, rose 2.3%, reversing earlier losses.

As the election looms, weak economic growth and high unemployment are among the key issues on voters' minds.

The planned break-up of Anglo, which had already announced thousands of job cuts in South Africa due to lower metals prices, raises the prospect of more job losses. The main union federation COSATU said it would seek reassurances.

Wanblad said the group, which has a 79% stake in Amplats, remained committed to South Africa. "I can't belabor the point more that this is not actually leaving South Africa in any way, shape or form," he said.

Founded by gold and diamond baron Ernest Oppenheimer at the peak of the World War One in 1917, Anglo American has been synonymous with South African mining for decades.

It employs roughly 45,000 people in South Africa, though Amplats has announced plans to cut 3,700 jobs while Kumba plans 490 job cuts.

(Reporting by Felix Njini; Writing by Silvia Aloisi; Editing by Jan Harvey)