WASHINGTON/MOSCOW -- There is growing speculation that Iran could be providing funds to North Korea and enabling the rogue nation to continue nuclear and missile development, despite a slew of heavy international economic sanctions.

"Iran just test-fired a ballistic missile capable of reaching Israel. They are also working with North Korea," U.S. President Donald Trump claimed in a message posted on Twitter Saturday.

Iran and North Korea have not announced any cooperation in nuclear and missile development, but U.S. and European intelligence agencies see it as an "unquestionable fact," Western diplomatic sources said.

"The second stage of North Korea's Hwasong-14 missile is similar to the upper stages designed for the Iranian space launch vehicles," Jeffrey Lewis, director at the James Martin Center for Nonproliferation Studies, wrote in an article published July. The American nuclear nonproliferation expert came to the conclusion after watching images of the missile launch released by the North Korean news media.

Growing suspicion

Iran and North Korea had cooperated in the development of missiles and other military technologies over a long period starting in the 1980s. Iran's Shahab-3 medium-range ballistic missile was developed based on North Korea's Nodong, it has been said.

Experts have pointed out that the missile, which Iran claimed it successfully tested Saturday, shares many resemblances with North Korea's Musudan intermediate-range ballistic missile.

There is a high possibility that Iran and North Korea are still working together because they can get huge mutual benefits by cooperating, Yuri Fedorov, an expert on the Russian military, said.

Iran's 2015 agreement with the U.S., the European Union and others has restricted its ability to develop nuclear technologies. However, Tehran would be able to continue and even accelerate its nuclear development by conducting studies in North Korea, Fedorov noted.

North Korea would also benefit from cooperating with Iran. The Middle Eastern country has boosted natural resources exports following the 2015 agreement. Pyongyang, struggling under the heavyweight of economic sanctions imposed by the U.N. and the U.S., could obtain much-needed funds for nuclear and missile development from Iran. Furthermore, North Korea could gain access to Iran's uranium enrichment technology, as well as technical information at U.S. and European research institutes via Iranian researchers.

Russian angle

There is speculation that Russia has given a tacit nod to cooperation between Iran and North Korea, seeing stronger relations between the two countries as helping to counter the U.S.

According to Western diplomatic sources in Moscow, Kim Yong Nam, chairman of the Supreme Assembly of North Korea, took flights via Russia when he visited Iran in August with many military experts in tow.

The U.S. has not attacked North Korea because Washington knows Pyongyang has nuclear weapons, Russian Foreign Minister Sergey Lavrov asserted in a television interview aired Sunday. He also repeated comments that could be interpreted as defending North Korea's nuclear development efforts.

Russian President Vladimir Putin wants to use the North Korean conflict as a leverage to chip away at the unipolar world controlled by the U.S., Fyodor Lukyanov, a well-known Russian commentator on diplomatic issues, said.