Azerbaijan's Foreign Ministry on April 6 announced the expulsion of four Iranian Embassy employees amid a further escalation of tensions between the two neighbours.

Earlier in the day, Azerbaijan arrested six men who it claimed were linked to Iran's secret services. Baku alleged that the men were plotting to "set up a 'resistance squad' aimed at establishing a Shari'a state in Azerbaijan through armed unrest and violent overthrow of Azerbaijan's constitutional order".

Elaborating on the expulsions of the embassy staff, the Foreign Ministry said it summoned Iranian Ambassador to Baku Seyyed Abbas Musavi and informed him that "four employees of the Iranian Embassy were declared persona non grata". This, it said, was "due to their activities... incompatible with diplomatic status."

The four were given 48 hours to leave Azerbaijan, the ministry confirmed in a statement.

Earlier this week, the Azerbaijani State Security Service arrested five people over the attempted killing of MP Fazil Mustafa, saying the ringleader was working for Iran's secret services.

On April 5, Iranian MPs put out a statement strongly criticising Azerbaijan for opening an embassy in Israel, an arch-enemy of Iran, and threatening Baku with "many negative political consequences." Tehran described the embassy move as an action contrary to the interests of the Palestinian people and their rights. The lawmakers called on other Muslim countries to "strongly condemn" Azerbaijan's action in setting up a diplomatic mission in Israel.

In late January, Azerbaijan shuttered its embassy in Tehran after a gunman burst into its grounds and killed the embassy head of security. Baku has blamed the attack on the Iranian secret services. Iran said it was committed by a lone attacker who had personal grievances with Azerbaijan in relation to the disappearance of his wife.

Azerbaijan has also accused Iran of supporting Armenia in the longstanding conflict over Azerbaijan's breakaway Nagorno-Karabakh region. Baku is strongly supported by another regional heavyweight, Turkey, in its fight for the region.

Tensions between Iran and Azerbaijan also often arise when Tehran accuses Baku of fomenting separatist sentiments among Iran’s ethnic Azerbaijani minority, which accounts for around a fifth of the Islamic Republic’s population.