These plans met with little formal resistance from the US and other Western powers, although it was seized upon by critics as evidence of the potentially dangerous impact of the Joint Comprehensive Plan of Action. Even so, Russia turned out to be slow in actually beginning the transfer of the S-300 components, possibility because of sensitivity to such criticism and possibly out of concerns about the impact of the sale on Israel, a partner to Russia but a bitter enemy to Iran.

They delays led to Iran filing a lawsuit against Russia, alleging breach of contract. At the same time, the delays were repeatedly downplayed by Iranian officials who at various times claimed that the first delivery was either imminent or actually in transit. Finally, these statements were corroborated by Russia earlier this year, and the first of the S-300 systems was reported to have arrived in Iran in April.

Now the International Business Times reports that despite the initial delays and uncertainties, Russia has expressed commitment to delivering the rest of the weapons over the course of the next several months, and the Iranians are apparently in the process of withdrawing their lawsuit. The 900 million dollar contract calls for the delivery of five more of the S-300 systems, and multiple Russian officials have indicated that they plan to complete the deliveries by the end of the year.

However, there are also indications that this may not bring an end to discord between Iran and Russia on the topic of weapons sales. It has variously been reported that Iran is interested in buying a range of other weapons from Russia. Michael Rubin of the American Enterprise Institute even went so far as to tell a House Oversight Committee hearing earlier this week that Iran was on a “shopping spree” for arms among US adversaries such as Russia and Iran.

Rubin emphasized that last summer’s nuclear agreement leaves open the door for such sales by failing to specify the types of “offensive weapons” that Iran is still barred from purchasing. With specific reference to the Russian arms market, Tehran has expressed interest in tanks, fighter jets, and surface-to-air missiles, among other things.

But according to the International Business Times, Alexander Fomin, the director of Russia’s Federal Service for Military-Technical Cooperation has denied that Russia has any plans to make such sales, and has insisted Russia and Iran will not be discussing the issue in the near future.

Fomin also disputed Rubin’s account, saying, that tanks, missiles, and other “attach weapons” remain subject to UN bans, even in the wake of the nuclear agreement. But it is apparent that Iran aligns with Rubin’s account, and not Fomin’s, thereby opening up the door for future disputes over Russia’s unwillingness to participate in the desired weapons transactions.

Such disagreements speak to the broader complexities inherent in the relationship between Iran and Russia. The two have been mutually supporting the Assad regime during the Syrian Civil War, and last year the commander of the Iranian Revolutionary Guard Corps’ Quds Force personally helped to convince Moscow to begin bombing Syrian rebel targets in direct support of Iranian missions on the ground. However, numerous analysts have pointed to apparently different motivations for the two countries’ involvement, meaning that Russia could potentially be convinced to drop its support for Assad while Iran likely cannot.

This has been underscored in recent weeks as international negotiations sought to impose a partial ceasefire on the Syrian conflict. Russia reportedly participated in these efforts by at withdrawing a portion of its forces and limiting bombing raids to regions beyond the reach of the ceasefire. Iran, on the other hand, has been accused of massing IRGC and Shiite militant forces near Aleppo and pursuing offensive missions in defiance of the international efforts.

Although the S-300 shipments point to continued cooperation between Iran and Russia, the Syrian situation and the issue of additional weapons sales seem to highlight the current limits of that cooperation. In this sense, it presumably remains to be seen whether that cooperation expands or deteriorates, especially as Iranian foreign policy threatens to drag its allies and potential allies further into conflicts not only with the Syrian rebels but also with the government of Yemen and with Iran’s Arab rival, Saudi Arabia.

Previously, Saudi Arabia and Russia appeared to be pursuing cooperation of their own as Russia led non-OPEC efforts to negotiate with the Saudis, as OPEC’s leading representative, in order to freeze oil output in an attempt to stabilize prices. However, these efforts were derailed by the discord between Saudi Arabia and fellow OPEC member Iran, which escalated considerably in January after Iranian mobs attacked the Saudi embassy and consulate in support of the Iranian government’s accusations of anti-Shiite policies originating in Riyadh.

On Friday, the BBC called renewed attention to these tensions, in particular highlighting the ways in which they were expanding to involve Lebanon, where the prominence of the Shiite paramilitary Hezbollah makes the country into a significant battleground of Iranian and Saudi influence. The article emphasizes that Iran’s support of Hezbollah has convinced the latter to direct more of its efforts against Saudi Arabia, as opposed to just Israel, which was the focus of its original mission statement.

Some have suggested that the discord between Saudi Arabia and Iran in places as diverse as Syria, Yemen, and Lebanon effectively makes these and other conflict areas parts of one overarching sectarian war in the Middle East. To this it might be added that the currently uncertain roles of entities like Moscow threaten to make that conflict even more broad-ranging than the boundaries of the immediate region.