On 21 June, the HV attacked RSK positions at the Miljevci Plateau, located in the pink zone north of Šibenik. The TO forces in the area were subordinated to the 1st Brigade of the TO, and Lieutenant Colonel General Milan Torbica. The HV deployed 250 troops, elements of the 113th and 142nd Infantry Brigades, commanded by Brigadier Kruno Mazalin. The HV had infiltrated the pink zone along three routes—via Nos Kalik, across the Čikola river and by boat sailing upstream along the Krka River, during the night of 20/21 June. The fighting began at 5 a.m. as the HV force, deployed in 26 squads, captured six out of seven villages on the plateau by the end of the morning. At 8:00 p.m., the HV captured the village of Ključ, and all of the plateau. The advance created a HV-held salient south of Knin, several kilometres deep. It also led the RSK artillery to bombard Šibenik and HV bombardment of Knin in response, both on 22 June. The artillery fire progressively intensified until 23 June, while the RSK mobilised and counterattacked against the HV positions at the Miljevci Plateau. However, the mobilisation yielded only 227 additional troops, and the counterattack failed. An UNPROFOR assessment concluded the situation might deteriorate further and engulf all of the pink zones. To address the situation, UNPROFOR military commander Lieutenant General Satish Nambiar met with Deputy Prime Minister of Croatia Milan Ramljak and Chief of the General Staff of the Armed Forces of the Republic of Croatia General Anton Tus in Zagreb the same day, in order to discuss the developments on the Miljevci Plateau. Skirmishes continued on 24 June, accompanied by some artillery fire. Morale of the RSK troops plummeted though, causing a TO garrison based in nearby Trbounj to abandon its barracks.
Question: when did amphibious operations begin?
Answer: 20 June