Saudi Arabia and the U.S. have brokered cease-fires between Sudan's army and paramilitary Rapid Support Forces at talks in Jeddah, but there have been reports of violations by both sides.
"On Wednesday, yesterday, we adjourned those talks ... because the format is not succeeding in the way that we want," Phee told a House Foreign Affairs Subcommittee hearing on Thursday.
Sudan's army and the RSF have been battling each other for more than two months, wreaking destruction on the capital, triggering widespread violence in the western region of Darfur, and causing more than 2.5 million people to flee their homes.
Heavy clashes broke out in several parts of Sudan's capital on Wednesday as a 72-hour cease-fire, during which there were several reports of violations, expired, witnesses said.
Phee said that while the cease-fires have not been fully effective, they have allowed the transmission of urgently needed humanitarian assistance.
Phee called on other governments to join the United States in its sanctions, adding that the United Kingdom is considering doing so but that the European Union has been "rather slow" in making a decision to pursue sanctions. The United States is also in discussions with its Arab partners, Phee said.
The United States earlier this month imposed sanctions on companies it accused of fueling the conflict in Sudan, targeting two companies linked to the army and two companies tied to the rival paramilitary RSF.
U.S. Secretary of State Antony Blinken is considering elevating the role of U.S. Ambassador to Sudan John Godfrey, Phee said when asked whether the State Department would look at a special envoy role to advance U.S. policy in the country.
"The entire U.S. government is fully engaged in addressing this crisis given its impact not only for Sudan but for the region," Phee said.