UNITED NATIONS — President Barack Obama today declared the United States is a serious partner in combating global warming, telling world peers "we are determined to act."

"The journey is hard. And we don't have much time left to make it," Obama said in brief remarks at a high-level climate summit convened by U.N. Secretary-General Ban Ki-moon.

Obama sought to show U.S. resolve ahead of crucial talks in Copenhagen in December, when nations will try to reach a new global treaty to address climate change. He spoke at the start of a busy day of diplomacy ahead of the main United Nations General Assembly meetings.

"We understand the gravity of the climate threat. We are determined to act," Obama said. "And we will meet our responsibility to future generations."