(Newser) – Pro-Russian separatists in Ukraine have laid mines near the Malaysian Airlines Flight 17 crash site, making the "work of the international experts impossible," says government security spokesman Andriy Lysenko. Dutch crash investigators were again thwarted in their fourth day of frustration in trying to reach the site. Organization for Security and Cooperation in Europe observers went back to Donetsk following a discussion today with rebels at the crash site, but wouldn’t say what caused them to leave, the AP reports. Over the past 24 hours, fighting between rebels and government forces has killed 19 people as the latter tries to regain territory. Intensifying clashes are blamed for stymieing access to the crash site, which is still unsecured and littered with human remains nearly two weeks later. (Read more Malaysia Airlines MH17 stories.)