After meeting Chinese Foreign Minister Yang Jiechi at a regional summit in Hanoi, Clinton said she was pleased by the Chinese stance on the minerals, which are vital for high tech products, but said the world must find other suppliers.
She also urged China and Japan to cool fresh tension over an overlapping territorial claim in the East China Sea and offered to hold three-way talks, a proposal unlikely to be embraced by an increasingly assertive Beijing.
She then flew to China to meet its top diplomat, State Councillor Dai Bingguo, on the southern Chinese island of Hainan, where a U.S. spy plane made an emergency landing after colliding with a Chinese fighter in 2001 and its crew was held for 11 days.
That incident on the island -- also a base for China's expanding navy, including a planned new generation of submarines said to be capable of carrying nuclear missiles -- symbolises the growing friction between the two nations.
The United States has been uncomfortable about China's decision to slash rare earth export quotas generally and to cut shipments to Japan, with which it is embroiled in a territorial dispute over islands they both claim in the East China Sea.
While Chinese officials have said they will not exploit the high-tech ores used in lasers, superconductors, computers and other electronics for leverage, prices have spiked and firms are rushing to develop sources outside China.
"Minister Yang clarified China has no intention of withholding these minerals from the market," Clinton told a news conference in Hanoi after meeting Yang.
She said the United States, Japan, Europe and other allies would search for more sources of supply of the minerals.
"So, although we are pleased by the clarification we have received from the Chinese government, we still think that the world as a whole needs to find alternatives."
The two met on the sidelines of the East Asia Summit in Hanoi, which has been overshadowed by the Sino-Japanese squabbling over the disputed East China Sea islands, called the Senkaku islands in Japanese and the Diaoyu islands in Chinese.
Speaking to reporters in Hanoi, a senior U.S. official who spoke on condition of anonymity said: "We have made very clear to both sides that we want the temperature to go down."
Another area where Washington wants Beijing's help to ease tensions is the Korean Peninsula, where North-South relations sank to their lowest point in years with the March torpedoing of a South Korean warship that killed 46 South Korean sailors.
Seoul and Washington blamed the incident on Pyongyang, which denied responsibility.
U.S. officials said Clinton and Yang had discussed North Korea, next month's G20 meeting and Chinese President Hu Jintao's state visit to the United States in January, but provided no details.
Yang will visit the United States, probably in late November, to prepare for Hu's visit, the senior official said.
"We conveyed today the need for China to exert pressure and influence on North Korea ... to behave responsibly in the run up to the G20," to rebuild trust with South Korea and to honor its 2005 commitments to abandon its nuclear programs, he said.
Clinton said she had also discussed North Korea with President Lee Myung-bak of South Korea.
"This is a matter of great concern to all of us and we continue to urge the North Koreans to return to the negotiating table, pursue what they began in 2005," she said, referring to a North Korean agreement to abandon its atomic programme.
North Korea has twice conducted nuclear tests despite the 2005 agreement.
North and South Korea, which technically remain at war because the 1950-53 Korean War ended in an armistice rather than a peace treaty, exchanged gunfire across their heavily armed land border on Friday, the South's military said.
It was not immediately clear what was behind the skirmish, but in the past the North has carried out provocations around the time the South has hosted prominent international events.
