Nine days later, on 10 May 1975, the KRA continued its incursion by capturing the Thổ Chu Islands, where it executed 500 Vietnamese civilians. The Vietnamese military immediately responded to Kampuchean actions by launching a counterattack and removing Kampuchean forces from Phú Quốc and Thổ Chu, and then invading the Kampuchean island of Koh Poulo Wai. In June 1975, while on a visit to Hanoi, Pol Pot proposed that Vietnam and his country should sign a treaty of friendship and begin discussions on border disputes. However, those discussions never materialised, and the Kampucheans claimed that Vietnam turned down both offers. In August 1975, Vietnam returned the island of Koh Poulo Wai to Kampuchea and formally recognised Kampuchean sovereignty over the island.