NAIROBI, Kenya - Actor and human rights activist George Clooney made a quiet visit to a volatile border region between Sudan and South Sudan last week, ahead of testimony he's giving before a U.S. Senate committee on Wednesday.

Clooney met with South Sudan's minister of defence and a regional governor during last week's visit to the world's newest country, said Ezekiel Gatkuoth, a government official. He also met with President Salva Kiir, said an official who asked for anonymity because he wasn't authorized to speak to journalists.

Clooney made the dangerous crossing from South Sudan into Sudan's Nuba Mountains region where he met with residents forced to seek shelter in caves because of aerial attacks by Sudan's military, the official said.

Violence has flared along the Sudan-South Sudan border since South Sudan seceded last year, and some experts worry the conflict could grow. South Sudan shut down its oil industry this year after accusing Sudan of stealing its oil.

Wednesday's hearing before the Senate Foreign Relations Committee will examine the oil dispute and the limited access aid groups are being given to Sudan's southern regions. Aid experts say people who live in Sudan's Nuba Mountains will soon face a hunger crisis.

Clooney travelled to what is now known as South Sudan in January 2011 as the region cast votes to secede from Sudan. The vote was the culmination of a peace deal that ended more than two decades of civil war.

After that visit, Clooney helped found the Satellite Sentinel Project, which uses satellite imagery to track military movements and attacks in the hopes of bringing attention to and potentially heading off hostilities.

John Prendergast, the co-founder of the advocacy group the Enough Project, also travelled to South Sudan last week. Prendergast and Princeton Lyman, the U.S. envoy to Sudan and South Sudan, are also scheduled to speak at Wednesday's hearing.