ABOARD AIR FORCE ONE, Aug 15 (Reuters) - U.S. President Donald Trump said on Friday he would announce tariffs on imports of steel and semiconductor chips in coming weeks.

"I'll be setting tariffs next week and the week after on steel and on, I would say, chips," Trump told reporters aboard Air Force One as he headed to a meeting with Russian President Vladimir Putin in Alaska.

He said the rates would be lower at the start to allow companies to build up domestic manufacturing in the U.S., rising sharply later, following a pattern he has also outlined for tariffs on pharmaceuticals. He gave no exact rates.

"I'm going to have a rate that is going to be lower at the beginning - that gives them a chance to come in and build - and very high after a certain period of time," he said.

Trump said he felt confident that companies would opt to manufacture in the United States, rather than face high tariffs.

Trump has upended global trade by imposing sharply higher duties on nearly all countries' exports to the United States, along with tariffs on specific sectors, such as automotive.

Trump in February raised tariffs on steel and aluminum to a flat 25%, but he announced in May that he would double the rate to 50% to boost domestic manufacturers.

It was not immediately clear if another tariff increase on the metals was in the offing.

Trump said last week he would impose a tariff of 100% on imports of semiconductors, but companies that committed to building up manufacturing in the United States would be exempt.

His remarks were made in tandem with an announcement that Apple would be investing an additional $100 billion in its home market.