Sen. John McCain on Tuesday praised the "brave fighters" battling the forces of Syrian President Bashar Assad and renewed his call for the Obama administration to move aggressively militarily to aid the opposition.

In a series of stops in the Middle East, the Republican lawmaker and former presidential candidate quietly slipped into Syria on Monday for meetings with commanders from the rebel forces and traveled to Yemen on Tuesday to sit down with President Abed Rabbo Mansour Hadi.

"Important visit with brave fighters in (hash)Syria who are risking their lives for freedom and need our help," McCain said on Twitter on Tuesday.

Gen. Salim Idris, chief of the Supreme Military Council of the Free Syrian Army, accompanied McCain on Monday as the lawmaker traveled inside Syria, the first U.S. senator to travel to the country since the civil war began more than two years ago. McCain has been a forceful proponent of military action against the forces of Assad and a critic of President Barack Obama's handling of the situation.

McCain spent about two hours in Syria, crossing over the border from Turkey, and met with about 10-15 rebel commanders, Idris said in a telephone interview from inside Syria. His discussions focused on the fighting on the ground, the need for military assistance, humanitarian aid and medical care.

"We are peaceful people, we would like to see our country liberated from this dictatorship, liberated from this murder regime, and we would like to have the best relations with all the countries in the world," Idris said Tuesday in an interview with The Associated Press.

McCain, R-Ariz., a member of the Armed Services and Foreign Relations committees, favors providing arms to rebel forces in Syria and creation of a no-fly zone. He has stopped short of backing U.S. ground troops in Syria.

Spokesman Jay Carney said Tuesday that the White House was aware in advance of McCain's plans to travel to Syria. Carney declined to say whether McCain was carrying any message from the administration, but he said White House officials looked forward to hearing about his trip.

A State Department official said the department was aware of McCain crossing into Syrian territory Monday, but referred further questions to McCain's office. McCain spokeswoman Rachael Dean confirmed the Monday trip, but declined further comment.

Yemen's state news agency SABA reported that McCain met with Hadi in the capital Sanaa. The agency said Hadi told McCain that he welcomed Obama's decision to lift the ban on transferring Guantanamo Bay prisoners back to Yemen. Yemeni detainees are the largest contingency in the U.S. prison facility, comprising nearly 100 out of 166 suspects held there. The senator also took a walk through the Old City in Sanaa and some of its bazaars, according to the news agency.

The United States backed Hadi's ascension to power after protests forced longtime President Ali Abdullah Saleh from power early last year. Yemen's military was also assisted by the United States when it fought to regain control of areas in the south of the country that had briefly fallen to al-Qaida , which is active on the southern tip of the Arabian Peninsula.

"Very informative visit to (hash)Yemen today - another nation under threat by al-Qaida," McCain said on Twitter.

McCain's Syria trip took place at the same time as meetings in Paris involving efforts to secure participation of Syria's fractured opposition in an international peace conference in Geneva.

And in Brussels, the European Union decided late Monday to lift the arms embargo on the Syrian opposition while maintaining all other sanctions against President Bashar Assad's regime after June 1, British Foreign Secretary William Hague said.

Two years of violence in Syria have killed more than 70,000 people, according to the United Nations. President Barack Obama has demanded that Assad leave power, while Russia has stood by Syria, its closest ally in the Arab world.

Last week, the Senate Foreign Relations Committee voted to provide weapons to rebels in Syria, as well as military training to vetted rebel groups and sanctions against anyone who sells oil or transfers arms to the Assad regime.

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Associated Press writer Bradley Klapper in Paris, Julie Pace in Washington, Karin Laub in Beirut and Ahmed al-Haj in Sanaa, Yemen contributed to this report.