The United States asked China to require exporters to register with the government, amid other measures meant to provide greater assurance of the safety of their products. The requests came amid continuing problems with Chinese exports, including chemically spiked pet food ingredients and, as of Thursday, potentially poisonous toothpaste and toxic fish.

“The Chinese understand very well that any nation that does not create an atmosphere of trust with consumers and customers, they will be disadvantaged quickly on the world market,” Health and Human Services Secretary Michael Leavitt told reporters.

A Chinese Embassy spokeswoman would not comment other than to refer a reporter to previous statements made by government officials. Vice Premier Wu Yi, in Washington this week for high-level economic talks, warned against politicizing economic and trade issues.

The latest problems with Chinese products began in March, when imported pet food ingredients were found to be spiked with the industrial chemical melamine and related compounds. The contamination was blamed in the deaths of dogs and cats in North America and prompted a cascade of recalls, with the latest announced just Thursday.

The problems do not stop at pet food. For April, China was No. 1 in countries whose imports were stopped at the border by the Food and Drug Administration. The list includes filthy mushrooms, drug-laced frozen eels and juice made with unsafe color additives.

“Obviously, there is a problem in China. It keeps getting bigger and we keep seeing more problems in different realms,” said Chris Waldrop of the Consumer Federation of America.

On Thursday, even more potential problems were disclosed: The FDA said it was stopping all imports of Chinese toothpaste to test for a deadly chemical reportedly found in tubes sold elsewhere in the world. The testing will look for diethylene glycol, a chemical commonly used in antifreeze and brake fluid, spokesman Doug Arbesfeld said. The imports will be released only if they test negative for the chemical. The announcement came after reports that tainted Chinese toothpaste had been sold in Australia, the Dominican Republic and Panama.

The FDA also announced an importer was recalling 282 22-pound boxes labeled as Chinese monkfish that it distributed to three states. Two people from the Chicago, Illinois, area became ill after eating the fish, which FDA testing later revealed contained life-threatening levels of tetrodotoxin, a sometimes deadly toxin. Importer Hong Chang Corp. in California believes the fish may be pufferfish, which can contain tetrodotoxin.

Because of the discovery, the FDA issued a blanket warning to consumers Thursday night “not to buy or eat imported fish labeled as monkfish.”

“There is a harsh reality here: When it comes to food, ‘Made in China’ is now a warning label in the United States,” said Democratic Sen. Dick Durbin, who exacted a promise this month from FDA commissioner Dr. Andrew von Eschenbach and Chinese Ambassador Zhou Wenzhong that they would work toward a mutual agreement to improve inspections and the overall safety of food products and drugs traded between the two countries.

When asked Thursday if consumers could trust the safety of food imported from China, von Eschenbach told The Associated Press: “This is one area where we have an ongoing opportunity for continuous improvement, and that’s what we are going to pursue.”

Democratic Rep. Rosa DeLauro, chairwoman of the House Appropriations agriculture subcommittee, pledged to hold a hearing to examine Chinese imports.

“This is becoming a day-by-day event. We got monkfish — poisonous pufferfish — as just yet another example. And they’re investigating toxic toothpaste imported from China. This is truly out of control,” said DeLauro, who criticized the administration’s food and drug safety requests to the Chinese as nonbinding.