WASHINGTON - Barack Obama's plan to build up U.S. forces in Afghanistan while keeping perhaps 50,000 troops in Iraq has triggered a deep rift among anti-war activists.

The split served as a reminder of the difficult task facing the presumptive Democratic nominee as he tries to broaden his appeal.

The Illinois senator wrapped up three days of tours and talks in the war-ravaged nations Tuesday, stressing in a news conference that the "situation in Afghanistan is perilous and urgent" and that "we should not wait any longer" to provide additional troops.

In Iraq, Obama won a tacit Iraqi endorsement of a plan to withdraw U.S. combat troops in 2010, but he also said that he backs leaving a residual force in Iraq to help train military personnel, provide security for U.S. interests and thwart terrorist threats. The residual force might total up to 50,000 troops, his campaign advisers said.

Some hailed Obama's trip as a breakthrough.

"So far the trip has been out of the park. It's an enormous moment," declared Eli Pariser, executive director of MoveOn.org, which supports Obama. He hedged about Obama's troop commitments, however; he said he wasn't fully aware of Obama's call for a residual force in Iraq and was trying to get a sense from MoveOn members on their views about Afghanistan.

Sister Simone Campbell, executive director of NETWORK, a Catholic social justice lobbying group, was less enthusiastic. "It was a significant step forward," she said, "but it was only a step."

Ian Thompson, lead organizer in Los Angeles for Act Now to Stop War & End Racism, found Obama's Afghanistan position similar to that of presumptive Republican presidential nominee John McCain.

"What this shows is that Barack Obama does not really represent any policy shift," he said.

Republicans say Obama supplied them with new political ammunition. Obama supports withdrawing U.S. combat forces within 16 months after becoming president, while McCain has called such fixed timetables artificial.

Rep. Heather Wilson, R-N.M., a McCain backer, said Obama has shown he's "frighteningly inexperienced. The difference is Senator Obama's [view] is based on the calendar, while Senator McCain believes the decision should be based on conditions on the ground."