UNITED NATIONS — Fuel is being shipped illegally from Iran to Houthi Shiite rebels in Yemen to finance their war against the government, and both sides are violating international law with their military campaigns and arbitrary detention of rivals, U.N. experts said in a new report. The experts painted a grim picture of a “deeply fractured” country sliding toward “humanitarian and economic catastrophe” with no sign of victory by either side in a conflict that many view as a proxy war between regional rivals Saudi Arabia and Iran. In the 85-page report to the Security Council seen Friday by The Associated Press, the experts said the government and its coalition partners led by Saudi Arabia made “significant progress” on the ground against the Houthis in 2018 — but their aim of restoring the government’s authority throughout the country “is far from being realized.” At the same time, the panel of experts monitoring U.N.