By Alexandra Alper

WASHINGTON, July 29 (Reuters) - A month after President Donald Trump said he would allow U.S. companies to resume selling to blacklisted Chinese telecommunications giant Huawei , his administration has done little to clarify what sales will be permitted.

The lack of clarity on what U.S. firms can supply to the world's top producer of telecommunications equipment as long as it's on a so-called "entity list" is likely to cast a shadow over this week's U.S.-China trade negotiations in Shanghai.

Trump had pledged to allow the sales as a goodwill gesture to President Xi Jinping when the two met last month and agreed to restart talks to try to resolve their year-long trade war. China, for its part, agreed to restart large-scale agricultural purchases.

U.S. chipmakers cheered Trump's announcement, which administration officials clarified afterwards meant the government would issue export licenses in cases where there is no national security risk and where the items are "non-sensitive" and readily replaced by rivals.

But the department has yet to respond to any of a total of around 50 license requests from about 35 companies, sowing uncertainty in the industry and in Beijing.

"At this stage, there is mass confusion," said William Reinsch, a former Commerce official, adding that the plan for case-by-case decisions "maximizes the uncertainty."

The governments of the world's two largest economies have imposed billions of dollars of tariffs on each other's goods, slowing global growth and roiling markets.

U.S. Treasury Secretary Steven Mnuchin and Trade Representative Robert Lighthizer will meet with Chinese Vice Premier Liu He starting on Tuesday, the first face-to-face meeting since the two leaders met.

Many people close to the talks expect the topic of Huawei to dominate, along with the failure of Chinese agricultural purchases to meet expectations, taking time and attention away from the many deeper, longer term issues.

Trump hosted a meeting of seven technology CEOs last week to discuss Huawei and other topics, at which the executives expressed frustration at Commerce Secretary Wilbur Ross for not providing clear guidelines, Reuters reported.

"By making the meeting public, the U.S. was trying to send a signal, 'we’re moving on Huawei, we need you to move on agriculture'," said Wendy Cutler, a former U.S. trade negotiator and Vice President of the Asia Society Policy Institute.

PAR FOR THE COURSE

Many companies have halted sales to Huawei since the company was put on the entity list on May 16, while some have chosen to resume selling items made abroad. Some, including Intel Corp and Qualcomm, began pressing Commerce for carve-outs soon after.