The United States is "gravely concerned" by reports of excessive force used by Uganda's security forces against lawmakers and journalists in the northwestern town of Arua, a State Department official said on Monday after the country's opposition called on Washington to suspend military support to Kampala.

Five lawmakers, including opposition leader Robert Kyagulanyi, who is also a musician known by his stage name Bobi Wine, were arrested in Arua last month. Kyagulanyi, who is currently in the United States for medical treatment, and fellow member of parliament Francis Zaake say they were tortured while in detention.

FILE - Ugandan pop star-turned-lawmaker Bobi Wine FILE - Ugandan pop star-turned-lawmaker Bobi Wine attends the National Press Club in Washington, Sept. 6, 2018. FILE - Ugandan pop star-turned-lawmaker Bobi Wine attends the National Press Club in Washington, Sept. 6, 2018.

"We have received multiple credible reports of excessive use of force by the Special Forces Command (SFC), including abuse of members of parliament, journalists and others," the State Department official said in a statement. "The United States has made clear to the Ugandan government that such human rights abuses are unacceptable."

Ugandan President Yoweri Museveni, in power since 1986, on Sunday warned against foreign interference in Uganda's politics.

Washington is a major source of funding for Uganda's military, supplying hardware, cash and training. It has given equipment, money and intelligence for the military's hunt for Lord's Resistance Army warlord Joseph Kony.

Museveni has also received diplomatic support from Washington for deploying troops in international peacekeeping missions, including the fight against militants in Somalia.

Uganda's longtime President Yoweri Museveni attend FILE - Uganda's President Yoweri Museveni attends his inauguration ceremony in the capital Kampala, May 12, 2017. FILE - Uganda's President Yoweri Museveni attends his inauguration ceremony in the capital Kampala, May 12, 2017.

The State Department official said the U.S. embassy in Kampala had not worked with Uganda's Special Forces Command (SFC) since early 2016 because of human rights concerns. The U.S. relationship with Uganda was aimed at promoting stability and security in the region, the official added.

"It is now up to the Ugandan government to demonstrate a respect for its constitution and its citizens and conduct a transparent, credible and timely investigation into these events," the official said. "Any security personnel found to have used inappropriate force must be held accountable."

Kyagulanyi has gained popularity for his attacks on Museveni. He was charged with treason last month over the suspected stoning of Museveni's convoy in Arua. He denies the charge and says he was tortured in detention.

He arrived in the United States last week for treatment for his injuries. Representatives from the State Department met with Kyagulanyi on Thursday "as they have done in the past when he visited the United States," according to an official.