BERLIN, June 1 (Reuters) - Germany will supply Ukraine with the IRIS-T air defence system, Chancellor Olaf Scholz said, following pleas from Kyiv and German opposition parties to step up heavy weapons deliveries.

Scholz said Germany had been "delivering continuously since the beginning of the war", pointing to more than 15 million rounds of ammunition, 100,000 grenades and over 5,000 anti-tank mines sent to Ukraine since Russia invaded it on Feb. 24.

"Most recently, the government has decided that we will deliver the most modern air defence system that Germany has in the form of the IRIS-T," Scholz told lawmakers in the Bundestag, without specifying the exact model.

A security source told Reuters last month that Germany was considering supplying IRIS-T SLM medium-range surface-to-air defence systems to Ukraine.

Berlin's decision on the matter does not mean the air defence system will be delivered swiftly.

"It will take a while, months," Foreign Minister Annalena Baerbock told lawmakers, adding the system, made by arms manufacturer Diehl, had initially been intended for "another country".

Responding to critics who claim Berlin has not done enough to help Ukraine defend itself, Scholz told parliament his government had responded to the Russian attack with a "massive change of policy in Germany" by opting to send heavy weapons into a war zone.

Ukraine's requests for heavy weapons intensified in recent weeks when Moscow turned its fiercest firepower on the country's east after failing to take the capital, Kyiv.

Scholz said talks were continuing with Germany's partners on ways to further arm Ukraine against the Russian attack.

On Tuesday, the chancellor announced Germany would deliver infantry fighting vehicles (IFV) to Greece so the government in Athens could pass on Soviet-style weapons to Ukraine.