The attacker evaded the first security checkpoint on Street 14 in the heart of the capital on Tuesday before blowing himself up inside the "Green Zone", killing at least five people and wounding dozens.
It was the first attack targeting the heavily protected area that is home to many embassies and international organisations since a massive truck bomb detonated on May 31, killing or wounding hundreds.
Since then the Kabul government has ramped up security in the city, installing scanning machines, truck barriers and dozens of checkpoints to prevent militants reaching the diplomatic and downtown areas.
Twice "we made it clear (to the National Security Council) that we have a problem with Street 14 -- there is no proper checkpoint because there is no barrier, just a bunch of policemen," a Western embassy official said.
He said another concern was the ability to avoid the first checkpoint by accessing Street 14 through a mosque and a park. The interior ministry said Wednesday that was apparently how the attacker got inside the Green Zone.
"That one (checkpoint) was a worry because it comes towards the British and Canadian embassies."
While security along the perimeter of the Green Zone has improved since the truck blast outside the German embassy five months ago, with more personnel stationed at checkpoints, it remained "porous", the expert said.
"This morning nothing had changed at this checkpoint -- they are sticking to the same procedure," another Western security source told AFP.
Defence ministry spokesman Dawlat Waziri defended the security arrangements, saying "many attacks are thwarted every day".
"Let's remember there are five or six million people living in Kabul and it is impossible to search everyone," he told reporters.
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