Kenyan police have arrested a man suspected of belonging to a terrorist group thought to be responsible for a series of attacks in the country, one day after two explosions in the capital Nairobi killed one person and left more than 20 wounded.

Police said late Tuesday they detained a Kenyan man, along with a weapons stockpile that included 13 hand grenades, two submachine guns, and four pistols.

On Monday, unknown militants threw hand grenades at a bus stop and a downtown bar in separate attacks in Nairobi. Officials did not immediately link the arrest to Monday's violence.

There was no claim of responsibilty for the Nairobi attacks, but Somali insurgent group al-Shabab had warned that it would carry out attacks throughout Kenya in response to a military incursion into southern Somalia by the Kenyan army.

Kenya blames al-Shabab for a series of recent cross-border kidnappings of several foreigners, a claim al-Shabab denies.

Since entering Somalia ten days ago, Kenyan forces have carried out airstrikes and advanced close to al-Shabab-controlled towns but have yet to fight a major ground battle with the militant group.

On Tuesday, the U.S. rejected Kenyan claims that it was taking part in Nairobi's military operation against al-Shabab.

The State Department said the U.S. has helped Kenya build its border defense capacity for years, but added, “The United States is not participating in Kenya's current operation in Somalia.”

A Kenyan army spokesman said Sunday that so-called “partners” had launched airstrikes against al-Shabab, and indicated that one of those partners was the United States.

The Kenyan army spokesman also said the French Navy had shelled the al-Shabab stronghold of Kismayo. The French navy denied that claim on Monday..

Al-Shabab is fighting to topple the U.N.-backed Somali government and set up an Islamic state in Somalia. The group has lost ground to Somali government and African Union troops in recent months but still controls considerable territory in southern and central Somalia.