WASHINGTON - More than 1,000 foreign fighters are streaming into Syria each month, a rate that has so far been unchanged by US airstrikes against the Islamic State and efforts by other countries to stem the flow of departures, The Washington Post reported Thursday Citing U.S. intelligence and counterterrorism officials, the newspaper said the magnitude of the ongoing migration suggests that the U.S.-led air campaign has neither deterred significant numbers of militants from travelling to the region nor triggered a spike in the rate of travel among Muslim populations inflamed by American intervention.

‘The flow of fighters making their way to Syria remains constant, so the overall number continues to rise,’ a U.S. intelligence official was quoted as saying. U.S. officials cautioned, however, that there is a lag in the intelligence being examined by the CIA and other spy agencies, meaning it could be weeks before a change becomes apparent.

The trend line established over the past year would mean that the total number of foreign fighters in Syria has already exceeded 16,000, the Post said, noting that the numbers eclipses that of any other comparable conflict in recent decades, including the 1980s war in Afghanistan. US officials have attributed the flows to a range of factors, including what they called was the sophisticated recruiting campaigns orchestrated by groups in Syria such as the Islamic State, as well as the relative ease with which militants from the Middle East, North Africa and Europe can make their way to that country.

American officials also stressed that the stability of the flow is not seen as a measure of the effectiveness of an air campaign that expanded beyond Iraq and into Syria late last month. The latest estimates indicate that strikes in Syria alone have so far killed about 460 members of the Islamic State - the group that has beheaded two American journalists and two British aid workers - as well as about 60 fighters with Jabhat al-Nusra, an al-Qaeda affiliate.

Experts said the foreign fighter population is likely to grow significantly larger as the three-year-old conflict drags on. ‘I don’t think 15,000 really scratches the surface yet,’ said Andrew Liepman, a counterterrorism expert at Rand Corporation, a Washington-based think-tank,  who previously served as the deputy director of the National Counterterrorism Center.

Since the start of the U.S.-led air campaign, analysts have sought to track whether the bombings would discourage would-be fighters or serve as a rallying cry for Islamists. Liepman said the steady numbers could mean that neither has occurred or, more likely, that both have happened to degrees that offset one another.

The air campaign ‘has probably discouraged some people and encouraged others,’ Liepman said. He and others cautioned, however, that there are significant gaps in U.S. intelligence on the conflict in Syria, making it difficult to have a clear understanding of the scale and composition of the swelling population of foreign fighters.

The vast majority of those militants have come from other countries in the Middle East and North Africa, including Tunisia, which has sent more fighters to Syria than any other nation. Even so, more than 2,000 fighters have come from countries in Europe, carrying passports that would enable them to travel relatively freely in Western countries.

Many went to fight the government of Syrian President Bashar al-Assad and may pose no security threat beyond that country’s civil war, according to the Post. But security officials have expressed mounting concern over more recent arrivals who have fought with the Islamic State or al-Nusra, which has a cell near Aleppo established to plot attacks against Western nations.

The United Kingdom, France, Germany and other nations around Europe have taken increasingly aggressive measures over the past year to stem the flow of fighters to Syria, seizing passports, passing new antiterrorism measures and targeting suspects with stepped-up surveillance and arrests. U.S. officials have said that about 130 Americans have traveled to Syria or tried to do so.

Most militants entering Syria have done so through Turkey, a country that has recently sought to tighten control over its borders and root out Islamist networks that serve as pipelines for fighters from Istanbul into Syria’s civil war. U.S. officials said it could be too soon to see clear indications that such measures are working.

Moreover, Islamic State (IS) fighters are heading to the twin conflicts in Iraq and Syria on ‘an unprecedented scale’, according to a new United Nations report.

‘Numbers since 2010 are now many times the size of the cumulative numbers of foreign terrorist fighters between 1990 and 2010 - and are growing,’ the report by a panel of experts monitoring al Qaeda and the Taliban said.

The report, which has been submitted to the Security Council,  says fighters from more than 80 countries working with al Qaeda associates in Syria and Iraq ‘form the core of a new diaspora that may seed the threat for years to come,’ and that domestic terrorism could rise as fighters return to their home countries. The increasingly sophisticated use of social media is giving the Islamic State group a ‘cosmopolitan’ appearance, it notes.

But the report says it isn’t clear whether the group will rise through divisions in the wider al Qaeda network to claim dominance of the movement.

The Islamic State group has alarmed the international community with its recent, sweeping gains in Syria and northwestern Iraq and with a series of videos of beheadings of foreigners. The panel behind the report was set up to support the council’s al Qaeda sanctions committee. The UN has raised concerns that those swarming the conflict zones include people from countries that are largely free of terror activities.

The report said that fighters from more than 80 countries were traveling to Iraq and Syria, ‘including a tail of countries that have not previously faced challenges relating to al-Qaeda’. The UN further said that Ayman al-Zawahiri, the leader of al-Qaeda who expelled IS out of his organisation, ‘appears to be maneuvering for relevance’, the report added. The report finds that 15,000 people have travelled to Syria and Iraq to fight alongside the Islamic State (Isis) and similar extremist groups. They come from more than 80 countries, the report states, ‘including a tail of countries that have not previously faced challenges relating to al-Qaida’.