WASHINGTON (Reuters) - The United States is willing to negotiate with China on trade, but only if talks are serious, as previous attempts produced little progress, a senior U.S. official told Reuters late on Thursday as trade tensions between the two nations escalated.

FILE PHOTO: Containers are seen at the Yangshan Deep Water Port, part of the Shanghai Free Trade Zone, in Shanghai, China March 14, 2018. REUTERS/Aly Song

No formal negotiating sessions have been set, the official said. “There is ongoing communications with the Chinese on trade,” said the official, who requested anonymity to discuss the Trump administration’s trade strategy.

The official said Republican President Donald Trump, who has already sought $50 billion in new tariffs on China, will insist on “verifiable, enforceable and measurable deliverables” from China in any trade negotiations.

The comments came as Trump said late on Thursday he had instructed U.S. trade officials to consider $100 billion in additional tariffs on China “in light of China’s unfair retaliation” against earlier U.S. trade actions.

In a statement, Trump said the U.S. Trade Representative had determined that China “has repeatedly engaged in practices to unfairly obtain America’s intellectual property.”

The senior official said: “We’ve had a type of negotiation in different forums where China has made lots of different commitments that they haven’t followed through on.

“We don’t want to go down that path. But the president has been clear, the administration has been clear, we’re not trying to start a trade war. We’re simply trying to get fair and reciprocal treatments so we’re open to those conversations.”

The official said China had committed seven times to stopping forced technology transfers, a practice in which China allegedly seeks to obtain U.S. intellectual property (IP) through joint venture requirements, something that China denies.

“This president is not going to tolerate hollow commitments or refusal to change bad practices. And if the way that we effectuate that is through negotiations, that’s great,” the official said.

The Trump administration on Tuesday targeted 25 percent tariffs on some 1,300 industrial technology, transport and medical products to try to force changes in Beijing’s IP, sending world financial markets gyrating.

China retaliated the next day with a list of duties on U.S. imports including soybeans, planes, cars, beef and chemicals.