Hundreds of US-led airstrikes and daily multi-million dollar military operations have failed to stem the tide of foreign fighters entering Iraq and Syria, who continue to pour in by their thousands, according to multiple reports.

“The flow of fighters making their way to Syria remains constant, so the overall number continues to rise,” a US intelligence official told the Washington Post.

Concrete changes might not be apparent for weeks the official said, noting the natural lag between the actual situation on the ground versus the scenario painted by analyzed intelligence.

With US intelligence estimating that 1,000 foreign fighters continue to enter Iraq and Syria every month, the Post estimates that their numbers now exceed 16,000. That figure eclipses the number of foreign fighters who took up arms in any analogous conflict over the past decades, including the Soviet military campaign in Afghanistan.

#US intel says that despite the ongoing airstrikes, 1,000 new foreign fighters join #ISIS each month. — PaulaSlier_RT (@PaulaSlier_RT) October 31, 2014

The figures fall within the same ballpark of a recent UN Security Council estimate. According to documents seen by The Guardian, 15,000 people have travelled to Syria and Iraq to fight alongside the so-called Islamic State (IS, formerly ISIS/ISIL) 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-Qaeda.”

The figures mirror US intelligence estimates in September, which also reckoned some 15,000 foreign fighters had entered the region via 80 different countries.

Reflecting the Post’s assessment, the UN committee notes that overall 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 committee, tasked with monitoring Al-Qaeda, did not mention the 80 states by name, though the overall geographical diversity of the fighters was unprecedented.

Meanwhile, the reports sharply underscore how expensive US-led military efforts have so far proved ineffective at stemming the tide of the IS advance in Syria and Iraq.

The United States and its allies have carried out more than 600 strikes so far in Syria and Iraq, with the strategic aim of allowing moderate opposition forces some breathing space to regroup. Since August 8, the US has flown 6,600 manned and unmanned sorties in Iraq and Syria at an estimated cost of $8.3 million a day, Defense News reports. Last week, it was reported that 32 civilians have died so far in the strikes.

Earlier this month, the Associated Press, citing Pentagon data, estimated that the United States alone had spent approximately $1.1 billion on its military operations in Iraq and Syria since mid-June.

Pentagon spokesman Rear Admiral John Kirby said this week that the bombing campaign was disrupting IS, though he admitted that any major offensive “may still be a ways off,” the Post cites him as saying.

US officials have said that the uninterrupted flow of fighters is not a proper metric to measure the effectiveness of the airstrikes, which have killed an estimated 460 IS fighters since last month, on top of 60 more fighters from the Al-Qaeda affiliate Jabhat al-Nusra.

Officials have attributed the number and pace of foreign fighters arriving in the region to several factors, including sophisticated IS recruiting techniques, and the relative ease with which would-be jihadists can travel from the greater Middle East and Europe to Syria and Iraq.

The UN report similarly recognized "the terror and recruitment value of multichannel, multi-language social and other media messaging," being conducted by the Islamic State.

A recent CNN/ORC International survey found that 54 percent of respondents believe the strikes will degrade and destroy the military capability of IS. That figure is down from 61 percent in September.

Meanwhile, the number of respondents who support sending US ground troops to engage the jihadists was 45 percent - a seven percent jump over the previous month. A majority, however, still oppose putting boots on the ground.