A Russian convoy carrying aid for civilians trapped in separatist-controlled Luhansk in eastern Ukraine diverted from its agreed route to the Ukrainian border Thursday, drawing warnings from Kiev that it will be blocked “with all the forces available” unless its cargo is first inspected.

Ukrainian officials and their Western allies have expressed concerns that the 280-vehicle humanitarian relief mission dispatched from Moscow on Tuesday could be trying to smuggle military assistance and reinforcements for the struggling pro-Russia insurgency.

The Russian convoy deviated from the main route to Kharkiv, a major city in eastern Ukraine still under Kiev’s control, and headed southeast to Kamensk-Shakhtinsky, a Russian town just across the border from Ukraine, the Ukrinform news agency reported.

As the army-escorted convoy idled awaiting instructions from Moscow, Ukrainian troops captured the town of Novosvitlivsk, on the main road between the Russia-Ukraine border and insurgent-controlled Luhansk, the Kiev government said. That could allow Ukrainian troops to block the convoy if Moscow decides to send it into Ukraine without the agreement of Ukrainian authorities.

The cat-and-mouse game over the aid mission has intensified fears that Russian President Vladimir Putin may be preparing to invade eastern Ukraine to shore up the flagging separatist movement he is accused of instigating. Putin denies he is the force behind the rebellion, but many of its leaders have been Russian citizens, including special forces officers, and the armored vehicles and sophisticated anti-aircraft batteries in insurgent hands are also believed to have come from Russia.

Putin visited Crimea on Thursday and pledged to invest more than $19 billion in developing the strategic peninsula seized from Ukraine in February and annexed to Russia a month later. No country has recognized the border changes inflicted by force, but neither have Ukraine’s Western allies moved to reverse the seizure with any action stronger than economic sanctions.

In a speech to local lawmakers in Yalta, a Black Sea resort that lost half of its usual summer patronage this year after the annexation, Putin expressed a desire to build up Russia without “fencing itself off from the outside world,” the Kremlin news service reported.

Putin’s popularity has soared in Russia since the March 18 annexation of Crimea, home of Russia’s Black Sea fleet and a historically Russian area ceded to Ukraine in 1954 by Soviet leader Nikita Khrushchev. In spite of the economic sanctions that have boosted prices and cut off some Russians from travel to the West, most have responded with newfound pride to the Kremlin leader’s pledges to restore Russia to the global status it had during the Cold War.

The convoy standoff began Monday, when Putin announced he was sending aid to Luhansk, forcing Ukrainian President Petro Poroshenko to accept the mission or appear to be denying basic essentials to those caught in the embattled insurgent stronghold.