You call these rumors? Let's go through it.
Tens of thousands of foreign jihadists ending up in Syria and Iraq don't drop out of the sky, it's hard to dispute the fact that all these foreign terrorists who ended up in Syria and Iraq gained access to these lands through a neighboring country. That leaves us with the following countries that might have been used as a gateway for these terrorists to gain access to Syria and then indirectly Iraq.
5 possibilities
-Iran
-Iraqi airports
-Jordan
-Saudi
-Turkey
Using Iranian airports, It would be quite retarded for terrorists to travel to the unpopular destination Tehran airport and from there take a land trip through Iran's mountains, through the Shi'ite heartland of Iraq to end up in Syria or Iraq, it's a failed attempt going through enemy territory. There have been no reports about jihadists using this path.
Using Iraqi airports to end up in Iraq or Syria is another possibility, though Iraq does not really expect tourists from other countries except for Iraqis visiting their own country. They don't have the big number of travelers at Baghdad airport, the security is so tight that it takes hours to pass through it as the country has been at war with terrorists for the last decade. It would equal to a PKK leader landing at Istanbul to visit PKK inhabited territory. Also no reports about jihadists using this path.
Using Jordan, you only have to look at the Syrian map and it will tell you enough. Naturally if Jordan was the main point of entry for IS Jihadists southern Syria would be a major IS stronghold. But it isn't, there barely is any IS there.
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As for Saudi Arabia, the Saudi-Iraqi border side is all desert and empty, they would have had to go some 400KM probably without anyone in sight to reach some 'safe' IS controlled area. This is speaking from 2013 and early 2014 when IS was using a certain gateway to enter Syria. There have been no mass reports about Saudi being used by Jihadists to gain entry to Iraq and Syria. An 18 year old Saudi was captured in Iraq, he explained how he ended up in Iraq. It was to take a flight from Kuwait to Turkey, then to proceed to Syria by land.
Now we arrive to Turkey, Turkey's border has until recently been fully controlled by IS, only American airstrikes reversed their growth on the Turkish border in favor of the YPG. There as mass reports about jihadists using Turkey to enter Syria. There are many terrorists who failed in their attempts, they somehow all report that their gateway was Turkey, we are talking hundreds of individuals here, whoever goes to Syria to join IS somehow decides to choose Turkey. Was it impossible to control the Syrian border or did Turkey pull a blind eye to all of that?
Let's look at ammonium nitrate, the chemicals used by IS for IED's that killed thousands of people. For years Turkey exported this to Syria with IS as a buyer, the cover being that it was used for agriculture but we all know that's not the case. They only banned sales once it started targeting Turkey. Was Turkey unaware that this could be used for explosives or did they again pull a blind eye to that?
http://www.reuters.com/article/us-turkey-security-fertilisers-idUSKCN0YV1MD
Turkey has banned the sale of fertilisers containing ammonium nitrate, which can be used in explosives, after at least half a dozen car bomb attacks this year, the agriculture minister said on Thursday.
The move comes after 11 people including six police officers were killed on Tuesday in a car bomb attack on a police bus during morning rush hour in central Istanbul. Newspaper reports said the explosives used were fortified by fertilisers.
Aside from that we have US officials as high as the chief of staff who in 2014 said that Arab allies have funded ISIS, they did this before 2014 of course, nowadays who would dare to do such things? They would clash with the US and the world community. You can look hearing up yourself between Lindsey Graham and Martin Dempsey the chief of staff, I don't want to put tens of video's here. IS kidnapping some Turks and releasing them after doesn't prove much, that is a one time incident back then, (foreign) policy of a state is observed by long-term actions, not a one time incident. Every actor whether state or non-state will commit acts that go completely against their policy, that does not prove anything.
Point is there's enough evidence, you don't even need to go by officials, if you use some logic you'll know enough. Of course it is harder for you to admit to any of it given that it goes against your country's interest.