Islamic groups in the US fear an overlap between the end of Ramadan and the anniversary of 9/11 will lead to criticisms that Muslims are celebrating the 2001 terrorist attacks.

Islam follows a lunar calendar, so Ramadan begins approximately 10 days earlier every year. This year, Eid al-Fitr – the festival that marks the end of fasting – falls on or around September 11.

Some groups worry that the coincidence will increase suspicion and hostility towards Islam at a time when feelings towards their religion are already running high.

President Barack Obama has come under intense criticism for throwing his weight behind plans to build an Islamic community centre close to Ground Zero. Newt Gingrich, the former speaker of the House of Representatives, compared the planned mosque to "Nazis" erecting a sign next to the Washington Holocaust museum.

Americans Against Hate have accused organisers of one Ramadan event of "spitting in the face of Americans" because they had scheduled activities for September 12.

The Islamic Circle of North America decided against holding its Muslim family day on September 11 out of respect for victims and families. Founder Tariq Amanullah worked in the World Trade Centre and died in the attacks.

An ICNA spokesman, Naeem Baig, said: "We took the decision not to have it on September 11 because it is not a day to celebrate. We will be mourning the deaths of all those who perished.

"We wish it to be as close to Eid as possible. But we don't want it on 9/11. That would be insensitive, we had to think of that."

He said that some critics would deliberately misinterpret Eid celebrations as something sinister but that the misconception was mostly due to ignorance of Islamic festivals.

The Muslim Public Affairs Council, an advocacy group based in Los Angeles, is reported to have contacted police to alert them to the overlapping dates. Another has urged mosques to improve surveillance and security on their premises.

Ibrahim Hooper, of the Council on American-Islamic Relations, said it was up to each individual community how they celebrated Eid but they might change the nature of the public events because of its proximity to 9/11.

"I don't think people will change their religious observance. Eid is a prayer in the morning but they might do something about the funfairs and bazaars they hold to celebrate.

"We always tell mosques to revise their security when Islamophobia is on the rise. We have a whole industry of people searching for any excuse to bash Islam and this is one of them."

Haroon Moghul, the director of an Islamic outreach body, wrote: "If Eid falls on the anniversary of that day, it will be an especially difficult task for a Muslim in New York. Many of our congregations were hurt that day, either personally or through the loss of loved ones. Many good friends of mine rushed to ground zero to give aid and spent hours, even days, doing what they could for the victims and for the brave first responders who were badly wounded."