Facing what President Barack Obama has called a new phase of terrorism, U.S. officials appealed to Muslim Americans on Monday to fight harder against extremist ideology.

The Obama administration has defended Muslim Americans after attacks in Paris and San Bernardino, California, and the inflammatory rhetoric that came in its wake, while a parallel message to Islamic communities is gaining urgency: please help.

As Republican presidential candidate Donald Trump suggested banning all Muslims from entering the country on Monday, Homeland Security Secretary Jeh Johnson stood in solidarity with an imam and leaders of other faiths at a northern Virginia Islamic center.

Johnson would not comment on Trump's remarks but urged Americans not to vilify Muslims or throw a "net of suspicion" over an entire community.

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But he had also made another appeal, this one to Muslim communities.

"Terrorist organizations overseas have targeted your communities. They seek to pull your youth into the pit of violent extremism. Help us to help you stop this," he said.

Since the deadly Nov. 13 attacks by Islamic State followers in Paris and last week's California shootings, there has been a clear call from American officials for Muslims to help police themselves. 

Three days after the Paris attacks, Obama urged Muslims around the world to ask "very serious questions" about how extremist ideologies take root and protect children from the idea that killing can be justified by religion.

"To some degree, that is something that has to come from within the Muslim community itself," Obama said at a news conference in Antalya, Turkey, after the G20 summit. "I think there have been times where there has not been enough pushback against extremism."

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The president made a similar plea in Sunday's Oval Office address on counterterrorism efforts. On Monday, White House spokesman Josh Earnest said Muslims will have to speak out against radicalizing forces in their own community.

"We would like to see leaders in the Muslim community stand up and speak out more forcefully in terms of condemning these hateful, radicalizing messages that we see from extremist organizations," Earnest said at a news briefing.

Leaders of the Islamic center where Johnson spoke, All Dulles Area Muslim Society, who said they have been doing all they can since the Sept. 11, 2001, al Qaeda attacks on New York and Washington.

"There's more denunciations coming from the Muslim community – not just now but for 14 years. Mosques are speaking out left and right," said Rizwan Jaka, a trustee of the Virginia center.