From the suburbs of Los Angeles to the outskirts of Washington, DC, mosques around the United States are warily stepping up security in the face of growing fears about reprisals on American Muslims.

The increasing safety concerns described by American Islamic leaders – and the steps they are taking in response, including hiring armed guards - represent the flip side of the rising public anxiety about Islamic State-inspired terror after attacks in Paris and San Bernardino, California.

The call by Republican presidential contender Donald Trump to ban Muslims from entering the United States only amplified concerns about an anti-Islamic backlash at mosques and community centers, religious leaders and organisers say.

At least two mosques – one in Phoenix and the other in suburban Virginia – are working with the Department of Homeland Security to check up on the security their facilities provide for worshippers in recent weeks. Others report taking a range of steps, including hiring armed guards, because of fears that an American mosque could be a target for an attack.

"We are always concerned about lone wolf attacks," said Usama Shami, president of a Phoenix mosque that has been working with the Department of Homeland Security to review its security measures since the Paris attack last month.

Over the weekend, police arrested a 23-year-old man suspected of setting a fire at a Southern California mosque in what authorities are describing as a hate attack, following the massacre of 14 people in San Bernardino on December 2 by a Muslim couple, US-born Syed Rizwan Farook, 28, and his Pakistani-born wife Tashfeen Malik, 29. Authorities have not said if the suspect was motivated by the shooting.

That fire set on Friday at the entrance of the Islamic Society of the Coachella Valley caused no injuries. But it charred the building's stucco front entrance and left it littered with debris.

The FBI is also investigating an incident in Philadelphia in which someone drove past a mosque and threw a severed pig's head at it from a passing truck as a possible hate crime.

On Thursday, the Council on American-Islamic Relations, an advocacy group, was forced to evacuate its office on Capitol Hill after receiving a letter containing white powder. The note said, "Die a painful death, Muslims," CAIR attorney Maha Sayed said.

"Our fear is at a pretty high level at this time, given the anti-Muslim rhetoric going on," said Sayed.

"Fearful" security guards quit

Given the rising tensions, some mosques say they have struggled to hire and keep security guards. In Dulles, Virginia, a suburb of Washington with a large Muslim community centre, security guards abruptly quit after the San Bernardino attacks, said Rizwan Jaka, chairman of the All Dulles Area Muslim Society.

"Security guards resigned because they were fearful of getting hurt in the backlash," Jaka said. "People were concerned."

The mosque has now hired armed guards and the imam of the mosque, Mohamed Magid, said security had been increased for programmes in which children take part. "We are concerned about the feeling in the larger community about Muslims," he said.

Jaka said that after the San Bernardino shooting federal law enforcement officials had also completed a security assessment for the mosque.

At the East Plano Islamic Center near Dallas, Texas, Nadim Bashir, the imam, said the mosque had hired an armed security guard ever since the Paris attacks. "We're just trying to ramp up our efforts in the community and get a better name," said Bashir.

A mosque in Corona, California, which, like San Bernardino, is a working-class suburb on the dusty eastern edge of Los Angeles, has spent $10,000 over the past two weeks to increase security. It is now asking for donations from the congregation to defer that expense, Imam Obair Katchi told Reuters.

The Islamic Society of Corona-Norco has also put up a banner on its website denouncing the San Bernardino attack. The mosque has faced extra scrutiny after it emerged that Enrique Marquez, who supplied guns used in the San Bernardino massacre, had once attended.

"The Muslim community stands shoulder to shoulder with our fellow Americans in repudiating any twisted mindset that would claim to justify such sickening acts of violence. We encourage everyone to be extra vigilant," the mosque's website says.

Not all mosques see the need for new security. Mufti Ikram Ul Haq at the Rhode Island Masjid Al-Islam said the mosque there is relying on a police presence during prayer times.

"We have surveillance. We lock our doors and we have an alarm system," he said. Local police, Haq said, "have been increasing patrols around our places of worship, and that gives us enough sense of security."

The FBI will not release data on hate crimes for 2015 until next year. Some critics, including CAIR, say the official statistics undercount reported incidents targeting Muslims. For 2014, FBI data showed that out of 1,140 victims of anti-religious hate crimes, approximately 16 pe rcent were victims of an anti-Islamic bias.

"Anecdotally, there is no question that we have had something of a flood of anti-Moslem hatred and hate crimes," said Mark Potok, a senior fellow at the Montgomery, Alabama-based Southern Poverty Law Center, a civil rights group.