BEIRUT, Apr 25 (Reuters) At least 18 Kurdish YPG fighters and media officials were killed in Turkish air strikes on a headquarters of the militia in northeastern Syria today, the British-based Syrian Observatory for Human Rights monitoring group said. The YPG, a US ally in the fight against Islamic State, did not immediately give its own precise death toll. Turkey's military bombed the YPG and Kurdish militants in neighbouring Iraq early today, widening a campaign against groups affiliated with the outlawed Kurdistan Workers Party (PKK). REUTERS SDR 1429