It is no secret that the bulk of Ivanka Trump's merchandise comes from China. But just which Chinese companies manufacture and export her line is more secret than ever, an Associated Press investigation has found.

Since Ivanka Trump became an official adviser to her father, public information about companies importing her branded goods to the US has become harder to find.

Information that once routinely appeared in private trade tracking data has vanished; leaving the identities of companies involved in 90 per cent of shipments a mystery.

Even less is known about her manufacturers. Trump's brand, which she still owns, declined to disclose the information.

The deepening secrecy means it's unclear who Ivanka Trump's company is doing business with in China, even as she and her husband, Jared Kushner, have emerged as important conduits for top Chinese officials in Washington.

The lack of disclosure makes it difficult to understand whether foreign governments could use business ties to try to influence the White House — and whether her company stands to profit from foreign government subsidies that can destroy American jobs.

Such questions are especially pronounced in China, where state-owned and state-subsidised companies dominate large swaths of commercial activity.

"I don't know how much money she's making on this and why it's worth it. I think it's putting our trade policy in a very awkward situation," said Richard Painter, who served as chief White House ethics lawyer for George W Bush, and is part of a lawsuit against President Donald Trump for alleged constitutional violations.

Among the few publicly identified companies that shipped Ivanka Trump goods to the US in the past, the AP identified one owned by the Chinese government and another selected for special export subsidies — a possible violation by China of global fair trade rules, experts said.

Ivanka Trump's brand doesn't make products directly. It contracts with licensees who oversee production. AP asked Ivanka Trump's brand, as well as its clothing, footwear and handbag licensees for supplier lists. All declined to disclose them.

Abigail Klem, who manages Ivanka Trump's brand, said the company does not contract with foreign state-owned companies, nor benefit from Chinese government subsidies. However, she acknowledged that licensees might.

"We license the rights to our brand name to licensing companies that have their own supply chains and distribution networks," Ms Klem said in an email. "The brand receives royalties on sales to wholesalers and would not benefit if a licensee increased its profit margin by obtaining goods at a lower cost."

Michael Stone, chairman of Beanstalk, a global brand licensing agency, said lower production costs for licensees would ultimately benefit Ivanka Trump by freeing up money for marketing or lower retail prices, both of which drive sales. "The more successful the licensee is the more successful Ivanka Trump is going to be," Stone said.

In 2014, a Chinese province announced a three-year pilot program to reduce export costs for ten local companies through insurance subsidies and access to special funds.

One of them was Zhejiang Tongxiang Foreign Trade Group, which shipped at least 30 tons of Ivanka Trump handbags to the US between March 2016 and February, according to shipping records independently maintained by Panjiva and ImportGenius, companies that track global trade.

Details about implementation are unclear, but four trade experts said they appeared to violate fair trade rules. "These are clearly export subsidies," said Gary Hufbauer, a trade expert at the Peterson Institute for International Economics in Washington.

Shipping records also show that from 2013 to 2015, 45 tons of Ivanka Trump clothing were exported to the US by Jiangsu High Hope International, which is majority-owned by the Chinese government.

The brand pledged to avoid such ties now that Ivanka Trump is a White House adviser, but contends that its supply chains are not its direct responsibility.

"That seems to be skirting the ethical commitment pretty close or avoiding it all together," Mr Hufbauer said.

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It is unclear whether the brand currently has any supply-chain links to foreign government-owned or subsidised entities.

The companies that shipped Ivanka Trump merchandise to the US are listed for just five of 57 shipments logged by Panjiva from March, when she took on her White House role, through mid-September. US Customs and Border Protection has not yet supplied the missing information to AP.

There used to be more visibility. In 2014, 95 per cent of companies that shipped Ivanka Trump merchandise to the US were identified in Panjiva's database.

Two licensees — G-III Apparel and Marc Fisher Footwear — appear to have redacted information, according to Panjiva records. Neither responded to AP's questions.

Chris Rogers, an analyst at Panjiva, said less than a quarter of companies ask customs to withhold information. "A lot of companies have said, 'yes there might be a commercial disadvantage, but we want to be transparent,"' he explained. "'Why would we want to cover up the fact that we're working with this particular company?"'