* Diplomatic pressure building, delegates head to Cairo * More than 1,224 Palestinians killed, 56 on Israeli side * Electricity cut as Gaza's only power station k**cked out By Nidal al-Mughrabi and Ori Lewis GAZA/JERUSALEM, July 30 (Reuters) - Israeli fire killed at least 21 Palestinians in the Gaza Strip early on Wednesday as the Jewish state said it targeted Islamist militants at dozens of sites across the coastal enclave, while Egyptian mediators prepared a revised ceasefire proposal. Israel's Channel Two TV said progress was being made to achieve a deal in Cairo, where a Palestinian delegation was expected to arrive for discussions. Israeli tank shells pounding houses in eastern Jebalya in the **rthern Gaza Strip killed 13 people and wounded many others, health ministry spokesman Ashraf Al-Qidra said. Eight people, including five members of the same family in Jebalya, were killed in other strikes across the Gaza Strip.