Two key US senators angrily pressed China on Monday to crack down on firms blamed for counterfeit electronics that end up in US military hardware, warning such components could harm US national security.

Senate Armed Services Committee Chairman Carl Levin, a Democrat, accused Beijing of tolerating "a brazenly open market" in the city of Shenzhen in Guangdong province, which he described as the epicenter of the illicit trade.

Levin slammed China's embassy for denying committee investigators visas to travel to mainland China and quoted one official as saying the probe concerned "sensitive" issues that could be "damaging" to bilateral relations.

"What is damaging to US-China relations isn't our investigation, it's China's refusal to act against brazen counterfeiting which endangers our troops and our missions," he said at a press conference one day before a hearing on the issue.

Senator John McCain, the top Republican on Levin's committee, noted that counterfeit electronics -- used parts made to look new and sold as new -- had turned up at the US Missile Defense Agency, in a submarine-hunting helicopter, and in a military cargo jet.

"We can't tolerate the risk of a ballistic missile interceptor failing to hit its target, a helicopter pilot unable to fire his missiles or any other mission failure because of a counterfeit part," he said at the press conference.

The lawmakers spoke out one day before the armed services committee was to hold a hearing into the issue, with government investigators, officials from Raytheon, L-3 Communications and Boeing, as well as the head of the Pentagon's Missile Defense Agency, Army Lieutenant General Patrick O'Reilly due to testify.

Observers of US politics expect to hear more anger directed at Beijing as the 2012 US presidential election campaign heats up, against the backdrop of stubbornly high unemployment often blamed here on China's economic rise.

Levin said counterfeiting costs the US electronics industry an estimated $7.5 billion per year, and called for changing US military procurement rules to ensure contractors bear the burden of replacing such components.

He and McCain also called for overhauling regulations to ensure that contractors that discover counterfeit electronics notify the Pentagon quickly.

"We cannot allow our national security to depend on electronic scrap salvaged from trash heaps by Chinese counterfeiters," said Levin.