WASHINGTON, July 17 - The White House on Thursday urged Russia, pro-Russian separatists and Ukraine to support an immediate cease-fire in order to permit safe and unfettered access to the site of a downed passenger jet. "It is critical that there be a full, credible, and unimpeded international investigation as quickly as possible," White House spokesman Josh Earnest said in a statement. Earnest said the role of international organizations such as the United Nations and the Organization for Security and Cooperation in Europe (OSCE) may be relevant for the effort and said the United States will be in touch with affected parties in the hours and days ahead. "It is vital that no evidence be tampered with in any way and that all potential evidence and remains at the crash site are undisturbed," he said.