Police in Kampala seized ¡°substantial amounts of explosives¡± and suicide vests in raids on a suspected al-Shabaab cell that was planning an imminent attack, a Ugandan official said in an interview on Sunday.

Police arrested 19 people in the operation on Saturday, and Information Minister Rose Namayanja urged the public to ¡°remain vigilant¡± as Uganda continues its investigation into the planned attack.

¡°The operation is still going on,¡± Namayanja said. ¡°We just want to ensure that we exhaust all the leads so that there are no more terrorist cells.¡±

Ugandan authorities say they have increased security at hotels and other key sites, including Entebbe International Airport, since making the arrests.

Last week a senior al-Shabaab official said the Somali militant Islamist group would target Americans in New York and Washington and ¡°capture Kenya and Uganda.¡±

¡°Anything could have happened,¡± if the plot had not been foiled, Namayanja said.

She said the government believes the Kampala cell had links to al-Shabaab but did not provide evidence linking the two, saying only that al-Shabaab has attacked Uganda before.

The discovery of the alleged cell came as Kenya prepared to mark the first anniversary of an al-Shabaab attack on Nairobi¡¯s Westgate shopping mall in which 67 people were killed.

In 2010, al-Shabaab bombed sports bars in Uganda where people were watching soccer¡¯s World Cup on television.

Both Kenya and Uganda contribute troops as part of the African Union peacekeeping force battling al-Shabaab in Somalia.

The militant group has threatened more attacks since the killing of their leader, Ahmed Godane, in a U.S. strike earlier this month.

Al-Shabaab has been reasserting itself in recent days following the death of Godane and the appointment of his successor, little-known Ahmad Umar.

Last week, the group targeted two military convoys near the Somali capital, Mogadishu, and on Saturday militants gunned down a senior Somali national security officer in his car, according to local police and an al-Shabaab spokesman.