Can't access Bedrock FM from REST Gateway

I'm deploying a REST gateway that calls Bedrock Titan FM. It works as expected when the REST Gateway runs locally using sam local --start-api .... Running the REST Gateway on the cloud, as follows:

sam deploy -t sam.yaml --s3-bucket $BUCKET_NAME --stack-name bedrock-gateway-stack --capabilities CAPABILITY_IAM 

and invoking endpoints that call the Bedrock Titan FM, raises the following exception:

2024-04-18 16:26:18,682 ERROR [fr.sim.qua.BedrockAiResource] (sdk-async-response-0-0) ### BedrockAiResource.inoke(): Model invocation failed: {}: java.util.concurrent.CompletionException: software.amazon.awssdk.services.bedrockruntime.model.AccessDeniedException: User: arn:aws:sts::...:assumed-role/bedrock-gateway-stack-BedrockGatewayApiRole-37LTyaxmS5Gi/bedrock-gateway-stack-BedrockGatewayApi-UbY8COwJ3v6I is not authorized to perform: bedrock:InvokeModel on resource: arn:aws:bedrock:eu-west-3::foundation-model/amazon.titan-text-express-v1 because no identity-based policy allows the bedrock:InvokeModel action (Service: BedrockRuntime, Status Code: 403, Request ID: 0781f5c7-a0f9-4f5a-99b0-127c20dd69b2)

Here is the sam.yaml file (generated by quarkus-amazon-lambda-rest plugin):

  AWSTemplateFormatVersion: '2010-09-09'
  Transform: AWS::Serverless-2016-10-31
  Description: AWS Serverless Quarkus HTTP - bedrock-gateway-api
  Globals:
    Api:
      EndpointConfiguration: REGIONAL
      BinaryMediaTypes:
        - "*/*"

  Resources:
    BedrockGatewayApi:
      Type: AWS::Serverless::Function
      Properties:
        Handler: io.quarkus.amazon.lambda.runtime.QuarkusStreamHandler::handleRequest
        Runtime: java17
        CodeUri: function.zip
        MemorySize: 512
        Policies: AWSLambdaBasicExecutionRole
        Timeout: 15
        Events:
          GetResource:
            Type: Api
            Properties:
              Path: /{proxy+}
              Method: any

  Outputs:
    BedrockGatewayApiApi:
      Description: URL for application
      Value: !Sub 'https://${ServerlessRestApi}.execute-api.${AWS::Region}.amazonaws.com/Prod/'
      Export:
        Name: BedrockGatewayApiApi

Also, the Bedrock playground in the AWS console works as expected.

Many thanks in advance.

Nicolas