The United States provided Ukraine with portable weapons in the first phase of hostilities rather than ammunition and heavy artillery because it expected Russia to take over quickly, The Washington Post reported on Wednesday, citing a senior US defense official. 
 
Portable weapons, like air-defense systems Stinger and anti-tank systems Javelin, were expected to serve as underground resistance instruments, the newspaper said. 
 
In mid-March, Ukraine almost completely ran out of artillery ammunition for a few days, but their fight beyond expectations "caused a scramble" in the US, the report said, and prompted a boost in arms supplies. 

Read: US sends $450 million worth of weapons to Ukraine
 
The war started in Ukraine on February 24 in response to calls from the people's republics of Donetsk and Lugansk for help in countering the aggression of Ukrainian forces, which has been ongoing since 2014. The Russian Defense Ministry confirmed that the operation was targeting Ukrainian military infrastructure only and the civilian population is not in danger.

A couple of days before launching the operation, Putin had recognized the independence of the Donetsk and Lugansk people's republics after weeks of escalating shelling, mortar, sniper, and sabotage attacks by Ukrainian armed forces and ultra-nationalist and neo-Nazi groups in the Donbass region.

Russia had for months been warning of the threat posed against it by NATO's attempts to expand eastward, which happened simultaneously with an increase in NATO military activity along Russia's borders, and batches of lethal weapons being sent to Ukraine, prompting Russia to request security guarantees from the West. Washington failed to provide the guarantees.
 
In April, Russia sent a diplomatic note to all countries regarding arms supplies to Ukraine. Foreign Minister Sergey Lavrov noted that any cargo carrying arms for Ukraine will become a legitimate target for Moscow.